As I understand it, the Xbox One has a 360 API translation layer. There may be some nonportable assembly code around--there almost certainly is--but (speculative) most use of assembly I've seen in the real world tends to be developed alongside a "slow" C/C++ path which may not be so slow on the Xbox One.

It wouldn't be as purely simple as a recompilation, but it's a conceivable amount of work. Probably be more work than is worth it for many titles, but I'm surprised at least the Big Games don't have support.