It may be that other Python implementations use CPython's standard library, or at least part of it.
I think that PyPy in particular does this, but I'm not 100% sure. I know for certain that it uses pure Python implementation of some modules from somewhere. One program I took great pains to be PyPy compatible ended up being a lot slower in it, and it turned out to be that the built-in sqlite3 module has a C implementation in CPython that's faster than the pure-Python version even when runing in PyPy.
I think that PyPy in particular does this, but I'm not 100% sure. I know for certain that it uses pure Python implementation of some modules from somewhere. One program I took great pains to be PyPy compatible ended up being a lot slower in it, and it turned out to be that the built-in sqlite3 module has a C implementation in CPython that's faster than the pure-Python version even when runing in PyPy.