How are you sure typing would have made a difference to that piece of Python? Are you sure there is no troublesome code written anywhere in Java or C++?
Interesting analogy, but in my experience it doesn't hold up.
If you have a Python typing bug, it will crash your program almost immediately and you can quickly catch it and fix it. In contrast, C boundary checking issues often come up in relatively rare corner cases, and when they do occur, they often do not result in an exception / error especially if the memory is already paged and set to zero.
In production code, boundary checking errors in C occur all the time, but Python type bugs are pretty rare.
People can change unit tests. The benefit of this, is it's a kind of documentation. People can see the tests and know how it _should_ work, or they can not read them and if they create errors, they'll find out the hard way. Unit tests don't make a specification static, but they do reduce chaos.