Well, a trailer (or documentary) saysing “promised the throne” while he wasnt actually, I wouldnt see that as a huge historical flaw tbh. More of a more picturesque language 
I dont know if the viking age was over at the displayed point in time, but it seems you do. that would be consederably inaccurate, yeah.
I am not very good with times/dates though and mainly influenced by series, but in “the last kingdom” while they are called danes already, they are also considered vikings.
Apart from that, which is likely wrong or another time period, I thought vikings basically simply means to raid, so danish raiders could be considered vikings. But it seems I am also mistaken there.
Anyway, it is possible the trailer/marketing texts did not get the entire picture of whats displayed, i guess the full campaign will show. I believe the devs if they say they researched a lot.