If it's just the kernel and not the userland above it, I doubt it. Apple's open sourcing of XNU and Darwin was never an existential threat to Apple.
The question is how much third-party code there is in the NT kernel. There may even be remainders of the cooperation with IBM tainting the source code (Windows NT did have an OS/2 personality at some point).
Based on what researchers who had access to the Windows kernel are saying, Windows has a decent amount of edge cases and special conditions to maintain backwards compatability with old software. For example if some popular software from 2000 abused some implementation detail that has since changed, Windows will detect that specific program and simulate old behaviour for it.
I expect most of that stuff lives in the userland interfaces to the kernel, but at least Microsoft seems to consider those to still be kernel code.
If you think of kernel32 & friends, that's not actually considered kernel code. That is the "kernel" of Win32, which is mostly a userspace thing. The word kernel is just used in an overloaded way.
The question is how much third-party code there is in the NT kernel. There may even be remainders of the cooperation with IBM tainting the source code (Windows NT did have an OS/2 personality at some point).