It doesn't work against precompiled native extensions, but it does work against wrappers against separate native libraries, like a database driver or libpng or something like that. The C interpreter can interface with native code, and the JIT has special support for making those calls very fast [1] (it's not like JNI, for example).
So the only thing it doesn't work for is precompiled native extensions where you don't have these source code - so maybe a proprietary C extension. Do you get many of those in practice?
> So the only thing it doesn't work for is precompiled native extensions where you don't have these source code
Or you can't build it because you're on windows, or the extension was installed precompiled by the package manager (on Python, that's exactly one of the use case for wheels, I don't think wheels ship C code ever), or the extension is coded in non-C (which is very much explicitly supported by CPython: https://docs.python.org/3/extending/extending.html#writing-e...)
So the only thing it doesn't work for is precompiled native extensions where you don't have these source code - so maybe a proprietary C extension. Do you get many of those in practice?
[1] http://dl.acm.org/citation.cfm?id=2500832