There are enough background books around to prove this statement wrong. The dark mechanicus enjoys experimenting outside the tight box of mechanicum dogma. Thats why they meld flesh and machine, daemon and machine, experiment with xenos wargear and so on.
The valkyrie is in the rules. A chaos transport is not.Same way as the Guard does, I expect.
so.... why I cant use them?or in dread claws, drop pods, storm ravens, thunderhawks and warp magic.