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.