It's not only threads. It's also explicit in whether a function can modify an argument because it's type signature says this with the mut keyword.

Well the article is about writing a database, which is definitely going to need concurrency and communication via shared memory to deliver the performance that it needs to (which is why the original implementation was mostly C++ and the new one is Rust). But yeah, I think in general if you're running into issues with mutability in Python, you're either using Python for something it should not be used for, or you're just programming in a really weird way.