Haha wow that is so ridiculous it's awesome. Seeing both the Linux root and Windows root with Program Files living right next to usr, etc. is _wild_. Very cool hack!
I think it was Ygdrassil, or maybe a free distro off of a CD. Great investment of time as it got me into the oilfield doing tech support. Running Rocky 8 on a P1 Gen4 now, so come a long way from my 486/Dx2.
Of course you can also do a cursed setup with 2 partitions: windows on Btrfs and Linux on NTFS just to maximize the chaotic nonsense.
WSL 2 is a completely different beast, with Linux storing its file system in a virtual disk image. \\wsl$\ just gives you the illusion of being the same system, but that’s just a networked file system.