As the blog post points out within a decade of the 'great heathen army' and just four years after this game is supposedly set a majority of these vikings had converted to Christianity including prominent leaders among them. That's a very different picture than what the game portrays.
There is also a difference between what the characters say and believe and what the environment depicts - e.g. there's not a problem with the norse thinking that they're doing good but rather with how the game constructs places which reinforce that narrative. e.g. the uninhabited river valley, the lack of slave trading
Seems like the only way for this to work out at all is for the hero to go home after a year (ie before converting): grow old. The entire setting needs to be an old man telling young kids about the old glory and trying to convert them from Christianity. That is about the only way you can justify only showing those actions: his memory, and latter life surrounded by Christian means he wants to remember only those parts. But now you really need to get the player to realize that this his a romanticized past - the player must spend a lot of time seeing contradictions in the player (that is it becomes obvious some things didn't happen the way the player played them!). There are a lot more implications of this line that I can figure out in a few minutes.
So many things in the blog post I didn't know - but fascinating and makes you think.... it's like the Spaniards and Portugese would have adopted Native American religions at one point.
There is also a difference between what the characters say and believe and what the environment depicts - e.g. there's not a problem with the norse thinking that they're doing good but rather with how the game constructs places which reinforce that narrative. e.g. the uninhabited river valley, the lack of slave trading