It's one of those things that make sense if you understand the details.
Windows NT itself has had an architecture supporting environment subsystems for things like Win32, POSIX, OS/2 for decades. See https://en.wikipedia.org/wiki/Architecture_of_Windows_NT for more details. Later, it made it relatively easy to host Linux and Android too.
You can imagine they commonly called these things "subsystem for XYZ". So "Windows subsystem for Linux" made sense for those in the know.
Does sound weird outside of the world of NT though
