Vikings are cool for the same reason Sparta is cool - "badass" tough mythical society. They are manly men fighters who are seen as aspirational for some ideological groups. Other ideological groups don't care all that much about them, because they are far away in the past. The people who consider Vikings as myth cool are not the same as the people who dislike real history of British Empire.

Also, the historical details over how their societies functioned are not important in popular imagination. For many people, Vikings that are cool are not real historical Vikings, it is more of safe imaginary fantasy setup - kind of like Lord of the Rings, Witcher or Star Wars. I mean, people do then project fantasy on real world history, but that is more of accidental thing.