I know about compatability mode. It still doesn't take away from the fact that it doesn't run on a 16 bit subsystem like 9x it merely emulates this for older programs.
It doesn't contain 9x code.
Here it is from the horses mouth (so to speak):
'Windows XP is built on an enhanced Windows 2000 code base'
From:
MS Windows XP Technical Overview
They didn't just rip bits out of 9x and shove it with 2k code and say 'tada, here's windows XP'.