I'd consider it the same, personally. It's a social contract that you can absolutely break if you want, just with a bit of work.
Less work in Python than it is Java, but there are ways to get to them in Java, or hell, just modify the source and change it directly. By doing that, you're doing something the developer didn't intend and you shouldn't be surprised if it doesn't work at some point in the future. Same as I'd expect in the Python case.
Sure, I think the bar to modify is higher in Java for example but I can't really find fault in what you're saying because I'd never unknowingly modify a member with double underscores in Python.
Less work in Python than it is Java, but there are ways to get to them in Java, or hell, just modify the source and change it directly. By doing that, you're doing something the developer didn't intend and you shouldn't be surprised if it doesn't work at some point in the future. Same as I'd expect in the Python case.