It doesn't mean they are implementable atop NT/Windows kernel facilities, semantics, and behaviours, in which case it's an alternative implementation to maintain that foregoes integration.

While Wine's effort and compatibility levels achieved are impressive, they actually prove my point: compatibility is a constant fight, there are a ton of tunables (some automated), and it's still not perfect (which is what a VM gives you for essentially free).

There's more than "basic" syscall interface to it though, there's the whole implementation of features (that are GPLv2 licensed) e.g btrfs? iptables? ebpf? ptrace? hold my beer; run kernel-backed wireguard in WSL2? no problem; alsa? you betcha; real time facilities? sure! custom kernel with a missing feature like usb passthrough? be my guest; contribute to the linux kernel code? entirely possible. It's all there, today, and not at the mercy of MS's will or ability to implement this or that.