I think they're wrong. I think it soared because it was a great Mario game. The fact that so many people played it multiple is because it's great at multiplayer, too. But I think very few people made their purchase decision with "multiplayer" as the deciding factor. Most of the buyers would still have bought it without multiplayer, and it would still have been a stellar game.
Maybe I'm too diehard of a fan, but the lack of being able to jump on other players/even run into them made it feel like essentially a single player game that's being played side by side