Okay, I don't answer your question, but I do show that an alternate history is required to keep the other evidence for catastrophic asteroid impacts from being uncovered regardless of what geologists in your society think about the KT boundary. And if your story happens in an alternate universe, perhaps geology develops at a lesser pace and has not yet become precise and sophisticated enough to realize what the KT boundary means.
If the story happens on Earth in an alternate universe, then the probability that the humans will ignore the possibility of asteroids causing extinction level events will be lower than the probability of that happening in some other solar system specially designed to make the danger higher but the natives less aware of it. But the probability of them being oblivious to the danger can be made much higher than it is in our alternate universe.
So I suggest several historical changes that should be necessary to make your alternate universe scoff at the idea of dangerous extraterrestrial bodies and their impacts on Earth.
I think that your premise that discovery of the KT boundary is what made humans aware of the dangers of asteroid collisions is an oversimplification.
The Alvarez impact theory was formed in 1980.
The article "Giant Meteor impact" by J. E. Enever was published in Analog March 1966 and reprinted in a couple of anthologies with six editions between 1968 and 1976.
As i remember, the article started with a premise familiar to science fiction readers in 1966, that an asteroid or giant meteor was detected heading for Earth and the space force is scrambling to deflect or destroy it. Then it is calculated that the asteroid will land in an ocean and everyone relaxes, because there is no need to prevent the asteroid from landing harmlessly in the ocean, right?
And then the article says wrong, an asteroid landing in the oceans would be an even worse disaster than the same asteroid landing on land, and proceeds to calculate and demonstrate why.
I remember there was a list of suspected astroblemes, or "space scars" on Earth, some of impressively large size. I remember the name of Vredefort in South Africa, which is still listed as the largest diameter impact structure on Earth at 160 kilometers, larger than Chicxulub at 150 kilometers.
The possibility of an extraterrestrial object striking Earth with devastating impact has been the subject of a number of stories for a long time.
The Year 4338; Petersburg Letters (1835) by Vladimir Odoevsky is set a year before Biela's Comet is predicted to collide with Earth.
"The Star" (1897) by H.G. Wells in which a star collides with the Sun.
When Worlds Collide (1933) by Philip Wylie and Edwin Balmer. A rogue planet collides with and destroys Earth.
In the Flash Gordon (1934) comic strip the rogue planet Mongo is on a collision course with Earth, but Emperor Ming says he will change the course to avoid collision.
In the Flash Gordon (1936) movie serial the rogue planet Mongo is on a collision course with Earth, but Emperor Ming says he will change the course to avoid collision.
In the When Worlds Collide (1951) move, a rogue planet collides with and destroys Earth.
In E.E. Smith's Lensman series entire planets were moved as weapons and smashed into enemy planets beginning with Grey Lensman (1939).
Astronomers first discovered lunar craters over 400 years ago, in 1609. For centuries astronomers debated the origin of lunar craters. The main theories were volcanic action similar to the volcanoes on Earth and giant impacts. The impact theory was proposed by Gilbert in 1893, Baldwin in 1949, and Eugene Shoemaker about 1960. Lunar probes and expeditions in the era of Project Apollo proved the impact theory about 1970.
The Mariner 4 probe discovered craters on Mars in 1965, Mariner 10 discovered craters on Mercury in 1974 & 1975, and eventually craters were discovered on all solar system bodies that were examined, except for those with strong geological forces and erosion from gases and liquids resurfacing them.
Meteor Crater, or Barringer Crater, in Arizona has had its origin disputed between volcanic and impact since 1891 and until the meteor impact theory was finally proven by Eugene M. Shoemaker in 1960. And as "Giant Meteor impact" shows, by 1966 there was a list of possible impact structures on Earth.
Early in the 19th century, scientists accepted that rocks sometimes fall from outer space onto Earth. The Wold Cottage or Wold Newton meteorite of 13 December 1795, so famous in fiction, was one of the events that convinced scientists that meteorites are real. And then of course it would be obvious to wonder what damage a vast meteorite the size of an asteroid could do if it hit.
On 30 June 1908 there was a vast explosion - equal to about 3 to 5 megatons of TNT - in the Tunguska region in Siberia, little noted in the outside world. There were seismic and atmospheric effects around the world, and the skies glowed at night for several days. But there was little association with the few reports of an explosion in Siberia. The first suggestion that it was extraterrestrial was in 1921, and the first expedition to the site was in 1927. The asteroid/comet theory gained acceptance within a few decades.
On 12 February 1947 a spectacular meteorite fall was observed in the Sikhote-Alin Mountains in Russia. Many tons of fragments were recovered, and the largest crater was about 26 meters (85 feet) in diameter.
In the Star Trek episode "The Paradise Syndrome" 4 October 1968, an alien planet is threatened with collision with a giant asteroid.
In the Star Trek episode "That Which Survives", 24 January 1969, Sulu says:
Once in Siberia there was a meteor so great that it flattened whole forests and was felt as far away as
Which shows how famous the Tunguska Event had become by 1968. Of course by the future era of Star Trek there could easily have been one or more later massive impacts.
On August 10, 1972, the Great Daylight Fireball was seen over the western USA. An object about 3 to 14 meters, or 10 to 45 feet, in diameter passed through the Earth's atmosphere and then returned to space, reaching as low as 57 kilometers or 35 miles.
On 15 February 2013 the Chelyabinsk Meteor exploded over Russia at a height of 29.7 kilometers with a blast estimated to be equal to 400 to 500 kilotons of TNT, 26 to 33 times the yield of the Hiroshima bomb. The object was approximately 20 meters (65.6 feet) in diameter and weighed about 13,000-14,000 short tons.
So in order for your world's scientist to believe that the KT iridium layer was terrestrial in origin, a lot of things will have to go differently in your alternate universe.
Discovery of the reality of meteorites must happen as late as possible.
Either the Tunguska Event, the Sikhote-Alin event, the Great Daylight Fireball, and the Chelyabrinsk Meteor don't happen, or they happen, but the areas they happen in are totally uninhabited by civilized people so no reports of them reach the outside world of science, causing people to know that city devastating impacts are possible.
The Euro centric scientists of your world will discount rumors from people they consider too backward to report data accurately. This probably means that the Russian expansion into Siberia starting in the 16th century and the American expansion into the trans-Mississippi west in the 19th century never happen.
Such historical changes will probably also help delay the popular science fiction idea of cataclysmic interplanetary collisions for decades or centuries.
There must not be any space race or any space exploration, which means that history has to be changed starting in the 1950s or earlier, which is no problem since the lack of exploration of Siberia and the American west must begin much earlier anyway. Thus there will be no space probes that prove that most objects in the solar system have many craters. And no proof that the Lunar craters are impact structures. And no great flourishing of planetary sciences in the later 20th century.
And above all, there can be no career of Eugene M. Shoemaker (1928-1997), who proved that Meteor Crater is a meteor crater and identified other impact structures on Earth, who proposed that Lunar craters are impact structures, who started searches for Earth-crossing asteroids that might potentially impact on Earth, discovering many, and who co-discovered Comet Shoemaker-Levy 9 in 1993, which was predicted to impact on Jupiter in July, 1994, an impact which demonstrated the almost unimaginable power of impacts and thus made a big impact on our way of thinking about impacts.
I suppose that those historical changes, plus some changes in geological theory, will be enough to keep the society in your alternate universes blissfully unaware of the dangers from cosmic rocks and their impacts until about AD 2018.
Sept. 6, 2018 I may add that in the Young Sheldon episode "Killer Asteroids, Oklahoma, and a Frizzy Hair Machine" March 29, 2018, 9-year-old genius Sheldon Cooper does a science fair project about detecting asteroids and avoid impacts, in the fictional year of 1989.
Of course that is fictional.