My understanding is that the idea for the character for Athelstan came after reading two accounts in the sagas and Saxon records of prior priests kidnapped by Vikings, assimilated in their society and that later went raiding with them.
One of them was captured, and crucified for being an apostate, as was written in the records
So it actually happened and they made a movie about it.
What’s the OP complaining about? That it doesnt fit his world view of Christians?
The claim is what you read, do you have a source that contradicts it?