Is this filtering hardcoded or is there any way to configure it e.g. via a Special: wiki page?
It's not configurable on-wiki (yet). Obviously not every Wikipedia or every wiki will find a category-based task lists feasible or useful, and certainly not with these exact categories. But the purpose of the extension is still, at this point, to help us learn which kind of tasks to deliver to first time Wikipedia editors in English, and how to do so effectively. Until we get that right, we aren't going to invest a huge amount in making a fully configurable version that would be used for permanent deployment elsewhere.