I specifically remember that I read that (it was a checkbox feature) about the ancient POSIX subsystem. The SFU/Interix system was a bit more capable, I think? I did install it at one time, but never really used it.

Edit: yes, that is in fact exactly what the first link you gave says: The POSIX subsystem was added as the POSIX standard had become very prevalent in procurement contracts. [...] This original subsystem was, I think it's fair to say, deliberately crippled to make it not useful for any real-world applications. Applications using it had no network access and no GUI access, [...] SFU contains a full POSIX environment, with a Software development kit allowing applications to be written that have access to networking and GUI API's.

I remember trying to use the Services for UNIX Version 1.0 Korn shell for a job. Coming from bash/zsh, I loathed it. I opted for Cygwin's bash ASAP.