I think GP is saying that unit tests for such dynamically-dispatching code will want to be able to inject older CPUIDs, so the compatibility fallbacks have test

I think GP is saying that unit tests for such dynamically-dispatching code will want to be able to inject older CPUIDs, so the compatibility fallbacks have test coverage. If CPUID is a native instruction, it's harder to inject a fake value. I'm not sure I find this compelling though. It's easy to make your own CPUID function that doesn't actually call "CPUID" in a test build.