How is does this different from publishing the instruction set, which was presumably already available (if it weren't, how would one write an application for it)?
What is less widely considered is that emulation is considered an implementation of a CPU. So e.g. research labs at universities have been told in no uncertain terms by CPU vendor legal notices to stop working on research that is emulating the x86 or ARM instruction sets. Now with the POWER ISA being open, everything about the ISA is fair game for research in emulation, soft cores, hardware, etc., which does put it in a space that only academic "toy" ISAs and RISC-V really sat in before.
I find it hard to believe and hard to enforce since emulators of various quality are practically everywhere (and not only for x86). QEMU?
IANAL, this is simply what legal folks are saying on this topic.