No it did not, it used Drawbridge OS research for Library OSes, which was integrated into Windows.

WSL 1.0 was a Drawbrige picoprocess.

Your confusion is natural among those that aren't into Windows programming.

Here are the links for the related information,

https://www.microsoft.com/en-us/research/project/drawbridge

WSL 1.0 architecture,

https://learn.microsoft.com/en-gb/archive/blogs/wsl/pico-pro...

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)

Proof? I couldnt find anything to that effect.