I was trying to post snippets from that video discussing things that were being mentioned in the thread at the time. For some reason though it didn't work.
That was before Mantle hit any titles and before DX12 was even announced. At this time there was only one new API Mantle. I think AMD were hoping it could become an industry standard, why not as DX12 was nowhere to be seen at this time. They say for that to happen Nvidia would need to adopt it, but they're not sure if they would adopt it. Unless im missing something? AMD have said many times that they hoped Mantle would be the spark for change and clearly it has as sometime later DX12 popped out from nowhere. Mantle can still survive after DX12 as long as devs continue to use it and develop for it. If we're to believe what Huddy says in the video, then the majority of devs in the beta program want Mantle to continue after DX12 launches.
EDIT
I think what he was trying to say is, ultimately there can only be one API that supports everything. Mantle is not going to be that API, though they may have thought that at the time because DX12 didn't exist then.
Thanks , that the sort of insight i was looking for i probably didnt explain myself quite how i was thinking in my head ( common issue for me sorry )