Yup, exactly correct. We'll be adding a feature soon to let you notify subscribers when you update/edit/add to an existing episode (if you want to update page by page or image by image or something with an episode/chapter).
As for sorting by date (i.e. sorting comics/series by date of last update)... that's in terms of how recently a new chapter/episode was added, and so would not necessarily reflect edited or updated existing ones. However, once the new notification feature rolls out, we'll definitely consider updating how the filters and sorting work as well to take that into account.