Maybe those are all people interested in the 3DS Mario Party .

Yes, Mario Party is that popular. The three 7th gen games sold over 19 million, which is as much as or more than series like Uncharted, Gran Turismo or Zelda did. Add in that Mario Party is strictly a multiplayer game (and thus played by far more people per copy bought than any of the three) and in terms of total players, it's probably significantly larger than either of the three. 

The prime reason that this particular video has so many views, is that boss battles are something completely new to MP9. Plenty of people have played MP at a friend's house, remember it fondly, and wonder what the video is when they see it suggested. 

Mario Party is immensely popular today, and has been popular for over a decade. You have people who remember the games fondly, and new players who are curious about it. When you then have a good video of a completely new feature in an otherwise very little changed series, it's no surprise it has many views. 

Ever since Youtube changed the way videos get views (before it was by just clicking the video, now you have to watch the entire video for the view to count), the automatic software that increase your views have disappeared, unless you are Vevo but that's a different story. 

That's why I don't believe this video got automatic views. However, it does seem strange that a video like this got so many views...it doesn't belong to a big official channel and the guy who posted the video only has 20k subscribers.

But well I guess this a case of crazy viral videos that for no reason get millions of views.

Really? that's weird, I thought Youtube wanted to avoid videos with thumbnails of a hot chick getting millions of views. Sad they changed it back. 

But wait, so that means when people use adblock it doesn't register the view?

Yes it's popular, but those particular numbers are deceiving, it's not that popular.