The point of the joke is not that the Python interpreter will change types mid-program on its own, but that you don’t have any real way of knowing if you’re going to get the type you expect.

Programs are messy and complicated, and data might flow through many different systems before finally being used for output. It can and often does happen that one of those systems does not behave as expected, and you get bugs where one type is expected but another is used in actuality.

Yes, most likely what would happen in Python is a TypeError, not actual output, but it was pretty clearly minor hyperbole for the sake of the joke.