Doesn't this mean AMD could make the same python API that targets their hardware, and now Nvidia GPUs aren't as sticky?

Nothing really changed. AMD already has a c++ (HIP) dialect very similar to CUDA, even with some automatic porting efforts (hipify).

AMD is held back by the combination of a lot of things. They have a counterpart to almost everything that exists on the other side. The things on the AMD side are just less mature with worse documentation and not as easily testable on consumer hardware.

They could, AMD's problem is that they keep failing on delivery.