i wouldn't declare that statement to be totally true in all cases.
all can be taken back to xp, but the computers/laptops especially loaded with proprietary hardware, designed for vista, will not have xp driver support.

i took a new compaq in on a trade for an xp dell, [customer didn't like vista].
it would run xp, but without any support for video, sound , harddrive etc. support.
this compaq was assembled just for vista