ROCm does not compile PTX IR to AMD assembly, if it did you'd be able to run nearly any compiled CUDA program on and AMD GPU without the source. ROCm is a source level compiler for CUDA (technically HIP is the actual compiler, but whatever), it allows you to a substantial fraction of the CUDA APIs. This notably also means that any Nvidia open source library that uses inline PTX assembly won't work, but fortunately AMD does have alternatives to many of the Nvidia libraries.

it's not, Nvidia just added support to OpenCL 3.0, AMD also continues to support it. Apple always does things its own way.

OpenCL 3.0 is basically OpenCL 1.0 rebranded, hence why NVidia didn't had any issue adding support for what they already had anyway.