That doesn't seem like a contradiction to the idea that "Windows subsystem" is (at least after WSL 1 and especially 2) a description for a functionality (i.e. running binaries targeting a different OS's interfaces), not an implementation.

No, as I explained, that's not what the actual subsystem architecture did. The binaries very much targeted Windows and did not target any other OSes. They weren't (say) ELF files targeting Linux, they were PE files targeting Windows, and you had to compile them from source with special flags to target those subsystems on Windows. You could not run those binaries on other OSes. The compatibility was at the source level, not at the binary level.