Even without the category system, if the user never ever goes back to rewatch a video they've seen, it could perhaps realize that the user doesn't want to rewatch videos they've already seen. Or if some videos or channels have a very high re-watch rate and others don't (presumably music channels vs., say, a random furniture restoration channel), it could tune for that. Clearly they're not separating the types, and it's not because they're not capable.