This is as close as UTC will ever get to an “It was all just a dream” ending. That’s not a spoiler, that’s a promise. Okay, it’s also a spoiler. It will never be all just a dream.
The implication in the final panel is that Morty ran away to hide in the fetal position behind those boxes, and when he grew larger, they fell out of the bookshelf. Reading it through again, though, I get this hilarious mental image of Morty, now back to full height, shoving the boxes off the shelf and crawling in there right before making his proclamation.