WSL1 doesn't even handle userspace. It just ships Ubuntu. And certification or no certification, Ubuntu is very trivially not POSIX-compliant. Perhaps you can m

WSL1 doesn't even handle userspace. It just ships Ubuntu. And certification or no certification, Ubuntu is very trivially not POSIX-compliant. Perhaps you can make it POSIX compliant, but there is no obvious way to do this; you would have to go point-by-point. I don't think that is any better than OSes that conform to some specific subset.