This is an example of someone not going to an online site anymore because their overhyped game didn't get the score they wanted. If their points are valid but you personally like the game a lot so you would give it a higher score, that doesn't mean that they are wrong.
The reasons they gave for a lower score (local multiplayer making the graphics and performance much worse overall, only change in difficulty was amount of damage you take and amount of health the enemy has, and bland bosses in general) were all good points, but the score is all determined by how much you think that affects overall game. The lack of exciting bosses and local multiplayer was enough to make it a 7.0, or only a "Good" game for the reviewer, while someone else may not have played the multiplayer mode causing them to not be affected by the performance issue and thought the dull bosses weren't that much of a problem, giving that person the impression that the game is better than it actually is.
The job of IGN is to assess the overall performance of the game, so they would need to analyze every single aspect of the game even if certain people would not use that part of the game. A personal note I have is Hyrule Warriors has little replay value, but that is to be expected with a Dynasty Warriors style game. I think the score should a bit be higher (around 8 or 8.5-ish), but 9.5 is a bit unrealistic.
EDIT: Made it more visually appealing by splitting up the giant block of text into paragraphs. Giant blocks of text aren't that fun to read through