The way WSL1 worked is actually quite interesting: The NT kernel always had the capability to run multiple subsystems, with Win32 only being one of them and there were subsystems available for OS/2, POSIX and later UNIX. WSL1 was pretty much a revival of that feature. So WSL1 is indeed somewhat like Wine, but making heavy use of some features built into the kernel. So yeah, no real boot process happening.
(Also it’s kinda stupid that the ‘S’ in WSL2 still stands for ‘subsystem’, despite not using that feature anymore.)