Not quite - CPython docs specifically say that some things are implementation-specific, and that other implementations are free to do them differently. For example, there's no guarantee of refcounting, and portable Python code has to assume that implicit cleanup is non-deterministic (which is observable with __del__).

The problem, rather, is that coders tend to ignore this, and write code that only works on CPython.

This is what I meant as the advantage of multiple implementations, by "CPython does whatever it wants" I meant that the power dynamics are almost exclusively in favour of CPython