IOW, the python implementation is the standard, and therefore any code that relies on an implementation detail in the reference implementation is, by definition, correct.
That's not quite strictly true, Python does have a documented "Language Reference": https://docs.python.org/3/reference/index.html
If there is a contradiction between the Language Reference and CPython then one, or both, of them needs to be updated and it's treated on a case by case basis.
If an alternative Python implementation follows the Language Reference but chooses different details outside it, that doesn't stop it from being "Python". Of course practically speaking most alternative implementations are incentivized to closely follow CPython.