Comment by ninkendo

It wasn’t the Unix subsystem, but was a subsystem… it implemented the various Linux syscalls and binary format directly and forwarded them to the NT kernel, similarly to how the various win32 syscalls call into the NT kernel. So WSL was a “peer” of the win32 subsystem in the same way SFU or SUA was. But it wasn’t implemented “via” SUA.

Then in WSL2 they just switched to using a VM, which is way more boring but probably less work to keep up with (I’m sure the Linux kernel has a lot of stuff that needs translating/implementing and it’s simpler to just use a hypervisor)