Precisely - macOS already is a perfectly good native BSD that (as you note) is UNIX® certified by The Open Group.
However, linux app binaries (notably docker images, which we're talking about here) need some sort of linux runtime, compatibility layer, or VM, in order to run on macOS.
The original implementation/version of WSL was an interesting POSIX-compatibility personality/layer for the NT kernel, but Microsoft seems to have decided that running a Linux kernel in a VM provided a better user experience (and probably made it much easier to track feature parity with Linux.)
However, linux app binaries (notably docker images, which we're talking about here) need some sort of linux runtime, compatibility layer, or VM, in order to run on macOS.
The original implementation/version of WSL was an interesting POSIX-compatibility personality/layer for the NT kernel, but Microsoft seems to have decided that running a Linux kernel in a VM provided a better user experience (and probably made it much easier to track feature parity with Linux.)