Shows what they know. File permission bits and umask are in fact supported in the Win32 API, but the only meaningful bit is the user-write bit (this dates back to MS-DOS days when files could be read/write or read-only). It's true that most permissioning is done by the ACL, of course. Cygwin has a whole set of logic for mapping Posix permissions onto Windows. and supports the umask in its shim layer.
I am reasonably sure that the u in umask stands for the old Unix kernel's u area, a part of the per-process state that could be swapped out with a process because it was only needed when the process was actually running. The C struct for this was struct user { ... } even though it was per-process and not per-user.