I suspect that from the usage in the code, it knows that there is a module foo and a submodule subfoo with a function bar() in it, and it can look directly in the file for the definition of bar().
It would be another story if we used this opportunity to mangle the submodules names for example, but that the kind of hidden control flow that nobody want in his codebase.
Also, it is not some dark arts of import or something: it is pretty standard at this point since its one of the most sane way of breaking circular dependencies between your modules, and the feature of overloading a module __getattr__ was introduced specifically for this usecase. (I couldn't find the specific PEP that introduced it, sorry)
