The Panopticon/What qualifies as a Multi-Doctor story? (Or rather, what doesn't?)

I would like to put up the argument that stories where the Doctor regenerates in reverse are not Multi-Doctor stories. By definition they do not feature multiple incarnations crossing paths in one story, but rather the Doctor briefly aging back to previous incarnations. So basically they're nothing but regeneration stories in reverse. So if The Tenth Planet isn't a multi-Doctor story, than I argue neither is Timeslip.