I think his point, and it's a valid one, is that Python has practically re-invented private members with special underscore handling anyway -- just more half-assed.
We may have zero evidence that private/protected makes software better but programmers, even in languages with no native support for it, seem to out of their way to do it anyway. Perhaps there is something to it afterall.
It was added more as a kind of "stop gap" for people who complained about the lack of private/protected attributes. That's why it is indeed half-assed. :) And that's also why hardly anybody uses it, especially the double underscore thing.
If Python developers were truly serious about everything being public there would be no underscored members.
Convention is something that doesn't exist in the language. You cannot mark properties as private or protected in Python so the convention is that underscore means private or protected. There are no getters or setters in Java so the convention is to use a method with a get/set prefix.
I would argue that such a common programming practice (enforced or not) should not be left to convention.