I agree that it would be weird to start with U3 and the page count was quite possibly altered along with the joke name.
If they do update the full U series, what else could we expect in the book if it were all adventures? We know it's going to have a nautical theme, so I'm trying, and struggling, to come up with another classic ocean/water-themed adventure. If it doesn't turn out to be "Tales from the Yawning Portal for water", I would have to believe my theory earlier in the thread would be reasonable - a water-based update to the ruleset, a bestiary, and the updated U series...