Animal Crossing 2.0
My guess? The just announced 2.0 update is full of content that wasn’t ready for the initial release date, and so was hidden from us (hense the stuff found in the datamines) until it was. And it took over a year to get to that point because the team was working on other stuff.
Oh, and some extra quality of life fixes, as well.
That explains the almost featureless Pier and Harv’s Island, then.