Maybe you should check their history, then answer me why they wouldnt.
Marketing is marketing, they are gonna try spin the most positive cases as possible. Even going above and beyond what it can actually do.
And just look at Mantle so far.
Its in consoles! Ups, its not in consoles.
Its open source! Ups, its not open source.
Its super quick and cheap to implement! Ups, its not so easy or cheap after all.
nVidia/Intel can support mantle! Ups, they cant support mantle.
Errm. Please point me towards the statements where AMD said these things.
They never said that it was in consoles; they said that it made porting from consoles easier. In fact they explicitly stated that it wasn't the same as the console APIs, to clear up the rumours that were circulating.
They never said that it was open source.
Where has it been demonstrated that using Mantle is hard or expensive?
AMD have said that Mantle could support any GPU with a compatible minimum feature set,
in theory, but since they haven't even launched it on their own platform yet they aren't working on multiplatform. They certainly haven't said "oh no, NVidia and Intel can't have it".
Don't conflate over-excited fanboys (on both sides) with what has actually been said.