The defensive potential of the shell
There's only medieval technology. These shells are strong. A human sized creature with a shell will be slow, but it will be very heavy and strong. It also has a real armor, suppose it can retract its head and limbs into the protective shell. Slolis will be difficult to kill. Your humans have medieval weapons, an axe or hammer cannot crack a Sloli shell, a crossbow arrow cannot penetrate it.
Shy swamp inhabitants won't be bothered
Slolis are peaceful tribes: they have their stories and primitive religion, but the other humans don't know all that. They are regarded as animals. Sloli settle near ditches, in swamp environments. They stay put in their territory and don't pose a threat. They don't understand other humans and humans are not able to learn the Sloli language. They don't even know if Slolis have a language.
Other humans have developed to medieval level, the Sloli thrive as (late?) stone age level hunter-gatherers in their domain. Because of their limited intelligence, Sloli stay behind in development. The species (or race) won't die out, because they found a niche. Slolis are shy. When humans enter their realm, the Slolis observe the humans, but not attack or confront them in any way. The humans will find poisonous snakes and crocodiles, before they see any Slolis.
Trouble in paradise: misuse of the shells, but it won't kill all Slolis
Sloli shell material is very strong and can be hollowed out, to craft shields. Before medieval times, other humans used to steal the shells by robbing Sloli graves. The Slolis bury their death on land, in forest ground. After a few years, when nature has done its work, the Sloli shell will be the only remaining part, and ready for use.
Some human settlements on the coast have started to develop ships, and trade. The Sloli's shell becomes a valuable asset, because it can protect against some of the new firearms (rifles) invented. At this point, humans will attempt to colonize the Slolis and breed them for their shells. Eventually, the arms technology level will surpass the point shields are useful. Sloli in captivity will loose their purpose for the colonists and die, or escape and return to the swamp.