Dire Bare
Legend
4e is only as rigid and limiting as you make it. You are right that the 4e tiefling fits in well with 3e cosmology, but I don't agree that your other assumptions are true.3E/2E fluff has room for the 4E appearance and background of both Tiefling and Genasi. 4E fluff however doesn't have room for the 3E/2E fluff, it's much more rigid and limited.
No offense to the author, but I do not see why I would limit myself that way.
If I run a 4e Planescape game and include Tieflings similar to how they were portrayed in the 2e Planescape line . . . . how does this go "against" 4e cosmology? Simple it doesn't. It expands 4e cosmology and is easy to do.
The only real limits on 4e are in your imagination and attitude, be creative and stretch those so called limitations.
Oh, and this goes for genasi too. And aasimar/devas.