No, it’s not a joke about the Inventory system, it’s an actual, Tetris-like game using the Diablo inventory pieces, courtesy of XY Games.
It’s fun to play even if you don’t like Tetris, since there are lots of quotes from D1 and D2 characters complaining about inventory space issues, plus hundreds of different objects, all of them using real Diablo 2 artwork. This includes sets and uniques; at one point I saw Tal’s Armor and gave a little yelp of joy. You’ll even see items you’ve never seen in the game (assuming you don’t play mods), since there are lots of 1×1 sized trinkets that are on the CD, but that aren’t used in the game. Example screenshot below.
The game looks ridiculously easy at first, since everything is a rectangle or a square. Beware though, since the pieces don’t drop down when you complete lines below them, so you quickly end up with floating obstacles all through the middle of the area. Also, entire pieces vanish when you clear a line, which is different from traditional Tetris. For example, if you clear the lowest line of a 2×4 polearm, the whole item will vanish, leaving a huge hole. Whether or not this is helpful depends on your playing strategy, but it’s definitely surprising, if you’ve played approximately a million hours of Tetris over the years, like me.