Even if they do, and that's a big if, since AMD has had a good headstart on designing hardware for DX12, as opposed to DX 11, it's not a big deal either way. You see the graph, based on the ONE benchmark/game out now, that shows the the Nvidia top dog, not designed for DX12, and the AMD top dog, designed for DX12, are not very far apart.
I foresee AMD improving, and Nvidia having Pascal designed for DX 12, and that there won't be a heck of alot of difference between the competitors at most price points. Personally, I think it's a helluva alot of noise being made on both sides about the issue right now, when there is one DX12 game and only a handful of DX12 games by the time Pascal drops.
I don't think either side's strident and zealous believers can really claim anything at this point.