People make the same mistake with Dan Brown books, too.
The point is that for the player character and allies to be killing large numbers of NPCs - well, killing small numbers and enslaving large numbers, more accurately - would itself constitute a nuanced representation of that period in history, this being very much what the historical viking raiders actually did. The major flaw Devereaux finds in the text is that it discards this nuance.
That Ubisoft's motivation in making the game they did is to make a lot of money, and only that, does not vitiate the value of the analysis with respect to history, both that of the 9th century CE and that of today. Devereaux makes clear enough, I think, that it's not about authorial intent, but about the meaning of the work in context - which is to say, the style of textual analysis he's using is the one that has been solidly doctrinaire for many decades now. To call this "overanalysis" seems to me to miss the point.
It's not to say anyone shouldn't enjoy the game as a game; Devereaux says right up front that he does enjoy it for its own sake, and isn't trying to detract from anyone else's enjoyment either. He's a historian knowledgeable of the period the game purports to represent, and so he analyzes it from that perspective. Anyone who does not want to partake of that perspective or that analysis is, it seems to me, at perfect liberty not to do so.