Cheers for the kind words, to expand on the basic idea (and if I didn't explain it properly) is the Devil that the Tiefling contacts is himself, 'from the future.'
I got that, and there's nothing in current D&D lore that would contradict your idea. I don't think you should invalidate your character concept based on a blurb from a decades-old AD&D book that has since been superseded many times over.
It's a really fun idea for a warlock, reminds me a bit of the short story
Other People by Neil Gaiman.