Did the boss fight not trigger the end credits? It should have once he goes red and explodes.
The level design was slightly meant to be confusing, it was designed so that you might run into an area, realize that it wasn't accessible yet and then have an 'aha' moment when you get an ability to let you access it. It's flawed in this case since there's only ever one way to go, in an ideal world I'd have made the map way bigger so that you can go between sections as unlocks happen but the jam was too short and I got too burned out to make a huge map and tons of content.