Just that, a solid nope. Normally I'd ask for some more information, but not only you said to just assume they can fly, you said it yourself that groundbirds are only capable of flying for relatively short distances and have a lifestyle that relies very little on flight. Much like ducklings raised by chickens alongside chicks don't suddenly become afraid ...
The recommended weight of a bird tracking device is 3% of the bird's body weight in order to not inhibit its mobility.
So I would say, no you could not have a Giant Teratorn be an airborne combat steed. but you could use a team of them as a rescue unit, like for example the firemen could have 2-3 of these birds to lift people out of precarious positions.