Optional mandatory locking

Windows NT file locking is - AIUI - entirely down to the "Windows personality" running on top of the kernel. The kernel in Windows certainly used to support POSIX-style file operations, so didn't have to enforce locking. The original design of Windows NT allowed for these "personalities", which were originally called subsystems. There was a POSIX one, an OS/2 one, and Windows itself.
Both the other subsystems are dead these days, as Windows has focused more and more on just being Windows.

My understanding was that Windows file locking was more a decision taken for backwards compatibility more than anything else. They decided to enforce the old DOS-style file locking (that you had if you loaded SHARE.EXE) when they went multi-user, as it was the cleanest and clearest way to do so and have existing applications understand what was going on if a file was locked.

As Windows NT was multi-user, they then went further and baked that file locking in to the OS components.

Of course, in practice none of this matters, except when you're trying to explain to someone why Windows inevitably requires a restart after a software installation/update...