A split is a moderator ability where we can remove certain posts from one topic and place them into a new topic in order to better facilitate a discussion that has wandered off the original topic.
A split is a split, regardless of who did it. The person doing it changes very little on the result. Indeed, it would have been ever better if a mod had done it, because it *did* originate in another thread where it was slightly off-topic and now if we reply here we can't see what was said in the other thread.
Back with the d-list, we didn't need mods to do thread splits. I don't see why mods would suddenly be an essential part in what the definition of a thread split is.