I haven't *totally* followed this, but it sounds like Blizzard spent a few weeks leading up to this teasing a new Diablo game, so everyone assumed it was going to be Diablo 4. Then they showed up with some shitty mobile app with microtransactions instead.
It's not really worth a lot of rage, but it'd be kinda like teasing a new Star Wars coming up, then announcing that it's just going to be a sequel to the Star Wars Christmas Special airing on Oxygen.