Windows NT was POSIX compatible.

The ZwCreateProcess kernel api can do fork() semantic.

AFAIK Windows kernel is advanced enough to implement fork if necessary. WSL 1 uses it.

Think at hypervisor level. Most VM solutions support some form of memory deduplication. VM level cloning wouldn't be too far from that model IMO.

This is exactly what VMware Horizon VDI does to create desktop VMs very quickly, called Instant Clone (aka VM Fork), and has been around for several years now.

With that, you don't even need to do deduplication after the fact, it comes for free. When a new VM is forked, it's memory and disk are copy-on-write from the source/parent VM.

Neither does GitHub and yet...

that's a non-sequitur. windows not being Linux at the same time that a Mac was also Unix is what created WSL 1, and the limitations of WSL 1 led to WSL 2.