They reference Elturel repeatedly, it's a city that was pulled from Faerun into Avernus by Zariel, also referenced. During the D&D adventure Decent into Avernus, it is possible for the player characters to have Zariel put the city back on Faerun. I expect the Tieflings escaped Avernus simply by fleeing to in while it was in Avernus and then tagging along for the ride to Faerun in the process.
