It's been established since 2008 that tieflings were turned into tieflings by asmodeus in an attempt to make them all one race, this did not go over well with fans. In descent into avernus the punishment for elturel was to be turned into tieflings and then dismembered by fiends. The only people that would have survived are warriors, like hellriders, and some mystics/priests, meaning most of the people that would survive would be tieflings. Karlach is a character from the descent that was turned into a tiefling by zariel, indicating that descent was another case where an archdevil was turning people into tieflings, but through contracts. This means all the fighting men in elturel would have been zariel tieflings like karlach.
There's nothing more about the people there and what happened to them because the setting doesn't even have a canon ending.
Larian tries to retcon this by saying that the tiefling refugees of elturel are ones that were "already there", but that they were "kicked out". This doesn't make sense as if the game is supposed to be a sequel to descent, the refugees would be formally human tieflings, most likely soldiers, fleeing the city after being returned to the material plane. Nothing about the game or the setting make sense