PTX does provide a low level machine abstraction. However you still target some version of hardware ( https://arnon.dk/matching-sm-architectures-arch-and-gencode-... ). However a lot of software effort has gone into it to make it look and work seamlessly.

Though AMD doesn't have the same "virtual ISA" as PTX right now there are increasing levels of such abstraction available in compiled flows with MLIR / Linalg etc. Those are higher level and can be compiled / jitted in realtime to obviate the need for a low level virtual ISA.