Without -nomangle, interrogate basically generates two versions of your functions: camelCase and under_score.
Thats why you can call nodepath.set_pos() and nodepath.setPos() for example.
Panda3D currently supports both versions, although the camelCase version is deprecated and will be removed in future versions. The problem is just that the whole direct/ source tree depends on camelCase, which is why its non-trivial to change.
The module builder has no such dependencies, and so I thought it’d be a good way to already encourage users to use the under_score versions of the functions.