"Could" wasn't the right word for me to use. Ofcourse they can, it's a matter of "would" they hardcode the engine. Seeing as MS announced they will be using the engine for their games, it is likely.
Considering Epics likes to have their engine running on as many platforms as possible (partially why it's so CPU heavy), I would doubt it. They're maybe platform specific paths for some functions or features within the engine with optimizations for Xenon (but pretty much any cross platform library, engine/RTE has that sort of thing)...