The way to parse WSL is that it's a <Windows (NT) Subsystem> for Linux.
Windows NT had environment subsystems as a key part of its architecture, with subsystems for POSIX and OS/2, originally[1], and even Win32 was conceptually a subsystem for NT, albeit a very important one.
[1] https://en.wikipedia.org/wiki/Architecture_of_Windows_NT#/me...