Kind of the opposite actually. PTX is in essence nvidia specific assembly. Just like how arm or x86_64 assembly are tied to arm and x86_64.
At least with cuda there are efforts like zluda. Cuda is more like objective-c was on the mac. Basicly tied to platform but at least you could write a compiler for another target in theory.
I think it’s because they reallly want to say “first!!” But have to add an adjective to be able to truthfully say it.