Me and a friend figured out how this happened. The game saves a "unlock next level?" state when you beat a level. if you beat a level you already beat, the game doesn't use up that state and you get to keep it. You must have went back to title screen and started a new save. I show a run where i did this to skip all levels but 1 in another thread, but didn't know the state was carried even to other save files!

If you win in a level, and then go to the latest level and immediately exit to hub, the next level will unlock. This means that you can play level 1 seven times instead of playing every level. I was going to do a couple runs trying to use it to get a good time but it feels like more hassle than its worth. I'm dubbing this useless-and-most-likely-slower-than-normal "skip", the Solo Skip. Here is a REALLY bad run i did to test the concept.