This is simply false. The Linux maintainers have little idea of how different libc implementers expose the interface they provide, and there are non-C languages using those ABIs as well. There is also full documentation for how to access any piece of ABI that Linux exposes without using C.
For example, here is a site that documents all Linux syscalls on x86_64, with the exact registers that they read their arguments from, and the registers they write their results in:
> Linux maintainers have little idea of how different libc implementers expose the interface they provide
This is literally false. A huge chunk of discussion on kernel mailing lists is exactly about this subject.
And, again. When OP used the word "Linux" they meant the entire operating system. The fact that this is also the name of the kernel project is irrelevant. Perhaps OP should've written "GNU/Linux" to satisfy mindless bureaucrats, but, I guess, OP hoped that this wouldn't be necessary.
OP explicitly talks about the system interfaces - i.e. the kernel. I don't see any sign that they are talking about the whole OS. In fact, if they were talking about the whole OS, the post would be nonsensical: any OS has a userspace implemented in some language, so any OS is 'pushing a language' in this limited sense, by definition.
Either way, the OP is clearly confused about what Linux-the-kernel provides vs Linux-the-OS. Their example of async IO is nonsense - async IO is built into the kernel itself, as are threads (which are not even available in libc, at least until very recently).
I will admit that I was "greatly exaggerating" (read: lying) when claiming that kernel maintainers don't know/care how libc uses their API. But the point is that, while the needs of libc obviously inform their designs, they are not directly writing or maintaining it, unlike virtually all other mainstream OSs.