To be honest, I would have preferred WSL1 which implemented the syscalls directly under Windows for the best possible integration, but they eventually just threw that away and switched to using virtual machines!
Given there's no "Linux specification" the spec is the actual code, nooks and crannies and all, down to nitty gritty details and implicit behaviours that end up indirectly getting relied upon. The only solution is to basically implement these in excruciating details, which may or may not possible depending on the Windows things being wrapped. Then the question of intellectual property, copyright, and licensing come in, as MS would have to ensure that it's a clean room implementation or be subject to GPL terms (cue the Oracle Java vs Android shenanigans).
So at some point it makes sense to just throw a VM at the job, cleanly separating realms, sidestepping any legal landmines, and ensuring perfect compatibility, especially as the whole HyperV infrastructure is quite solid and used throughout the MS ecosystem as a general strategy, e.g the Xbox uses HyperV to isolate games and apps.
Tangent: I wish (the VM part of) WSL2 would be available on Xbox, allowing one to run Linux while keeping the whole regular OS and features!
With Linux, all the syscalls and their bugs are documented, at least in the commit history, if not the man pages.
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.
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[1] never trust an OS that can't reliably be left alone for 24 hours without rebooting itself while you aren't looking
In any case: How do I get the version+configuration used on US military installations on my machine?