Actually I don't know it either. But WSL2 doesn't really start a lot of processes, like a desktop/server Linux distribution. It starts up only a minimal set of processes, and a shell.

dmesg looks like the kernel is booting up normally, so I guess they don't use some memory snapshots magic.

I guess they just tweaked the kernel and the hypervisor to start up fast. There is also no BIOS/UEFI delay.