What an understatement. The modern Linux syscalls are tightly coupled to the implementation of Linux. Special fds, io_uring, robust lists, etc etc …
There’s a reason why Microsoft said fuck it and run Linux in a VM for WSL2.
If you want Linux, just run Linux.
Edit: It's come to my attention that this particular instance of gatekeeping came from the Linux camp, not FreeBSD. Though you can find it in the FreeBSD community too, to be sure.
I'm not sure how relevant Microsoft is here given how much more different from Linux NT should be compared to FreeBSD. It's also kind of a different calculus for Microsoft in terms of it being worth the effort for what they wanted to do with it. Microsoft already has its own thriving ecosystem, the Linux support is just a bonus to attract people from a niche market. For FreeBSD the situation is more or less the exact opposite. The SmartOS people did it too, and even ported over KVM. But Joyent was acquired by Samsung and I'm not sure what happened to SmartOS after that.