__all__ is only relevant for * imports.

And please, just don't use * imports. It really doesn't save you much time at the cost of implicit untraceable behavior. If you don't worry about * imports, you don't need to add the __all__ boilerplate to every module.

This article is more about advertising a package called tach, that I suppose tries to add "true" private classes to Python.

But it doesn't actually enforce anything, because you still need to run their tool that checks anything. You could just easily configure your standard linter to enforce this type of thing rather than use a super specialized tool.