I think he's talking specifically about the Java and C++ code that accompanied that Python file. I'm sure he's seen bad Java and C++ :)
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.