it's a dubious claim to being "open" IMO. I've seen the "we've been told at the GPU14 Tech Day event that the Mantle API is open..." quote. first of all, I find the wording kinda weak as far as being a real confirmation. but beyond that, this would be a low level API designed from the ground up to work with a specific family of AMD cards. it kind of goes without saying that the more specific something like that is, the less compatibility it will offer. so although it may be "open" for nvidia to try to use it, there's a good chance it wouldn't be compatible at all with their architecture, and they'd probably be better off doing their own thing (aka a similar low level api for their cards, and even more fragmentation). that's something AMD will have to deal with as well, as it won't be compatible with their older cards, nor any future cards if/when they move to a different architecture
there's nothing really new about these concepts. but I think the PC getting away from a lot of these compatibility issues is what has helped PC gaming become more popular. if they start to pop up again, a lot of people might go back to consoles... maybe that's what AMD had planned all along
(I'm not an expert. I just play one on the internet)