Mmhm. One of the cool things I really like about Evan's stories - she takes the changes between editions as plot hooks instead of waving them away. She actually designed the story behind why succubi left the Abyss for the Nine Hells in 4e, and how they're now free agents in 5e.Not being a fan of the FR novels, I didn't know this bit of lore - that FR tieflings are a group of pre-4e tieflings that did the thing that made the 4e tieflings. It's interesting that the Asmodeus-linked tieflings are explicitly old-school tieflings who did a specific thing in a specific world. That has some ramifications for how they might address this in future material.
Like, it would totally be consistent for a PS-style sourcebook on planar adventurers to, at some point, say something like
Rubes from the Prime might think that all tielfings look the same and all are sworn to some lord of the nine or whatever, but that's just one of the many reasons not to trust rubes from the Prime. That might be the only story they're familiar with, but anyone who knows the planes knows there's more types of tieflings (or aasimar or genasi or whatever) than there are blades of grass in Arborea. If anyone imagines they all know how what a tiefling looks like or what it's story is, they're in for a rude awakening and a quick mugging when they wind up in Sigil.
....or whatever. And using that particular story lets them do that.
Another thing is that she actually introduced an old Planescape style tiefling in her books as well, a descendant of the Abyss, and the reaction that one of the infernal tieflings had when faced with them. So, its actually cannon that both the infernal-pact tieflings and the Planescape-style tieflings are running around Faerun.
The old Planescape style tieflings are cannon inside of Faerun (see above), and we have the DMG rules for making them. Can't get much more valid then that! Official lore plus rules for them.The one problem that I have is that WotC won't officially "validate" that methodology.