My only guess is they have a parallel skunkworks working on the same thing, but in a way that they can keep it closed-source - that this was a hedge they think they no longer need, and they are missing the forest for the trees on the benefits of cross-pollination and open source ethos to their business.
Even if AMD lagged support on CUDA versioning, I think it would be widely accepted if the performance per dollar at certain price points was better.
Taking the whole market from NVIDIA is not really an option, it's better to attack certain price points and niches and then expand from there. The CUDA ship sailed a long time ago in my view.