The POSIX subsystem is a box checking exercise. The reason the box was there isn't satisfied but the box was checked and Microsoft hoped that's good enough. If you need "a Unix" and they give you NT and circle the stuff about POSIX you don't go "Oh, perfect" you ask them to fix the requirements document so that you can have an actual Unix next time.
WSL is Redmond going OK yeah, here you go, an actual Unix.
It was a requirement on government contract tenders. If you didn't support POSIX, you didn't get the contract and the government would have continued using SunOS or AIX or something instead of Windows NT. Remember that POSIX was designed to make these OSes somewhat interchangeable - they wouldn't order something they knew was incompatible with everything they already had.
WSL is Redmond going OK yeah, here you go, an actual Unix.