Well that was because of the ten year celebration. And there gonna add more then a single enemy, as a ton of content from each game will go into the other. And it was literally told by Loki here on the fourms and the Re-Logic team on Twitter that it was gonna be a major update. And again, the ten year anniversary wasn’t considered a major update because it wasn’t adding a whole ton of content to the game, just 2 enemies and a cool painted world.
Show me where I - or anyone - said this is a "major" (eg 1.X) update?
A "major" update tends to have material impacts on core progression, features, and more. DD2/OOA was a 1.3.X update, this will be the same.