I suspect the reason people claim that is they are training ML models, which may take O(hours ~ days) to run anyways. In this usecase, C++ calling C++ is faster

I suspect the reason people claim that is they are training ML models, which may take O(hours ~ days) to run anyways. In this usecase, C++ calling C++ is faster than Python calling C++ slightly in O(seconds) is outweighed by the fact that Python is more convenient for the ML practitioner trying different models.