I don't know if such a theory has already been voiced, but still.
What if Niko couldn't figure out for sure if Jason was reborn or if Jason ended up in Elysium because Jason didn't go to the Underworld?
What if Jason ended up in Valhalla?
So Jason died in battle and was generally a wonderful man and warrior, so what if they decided to take him to Valhalla? Ordinary people were also taken there, so perhaps one of the valkyries could bring demigod from other pantheon to Valhalla?
It's also possible that Jason could have gotten there because Beryl might be the legacy of one of the Scandinavian gods (hey, we don't know much about her, so I'm not ruling out that possibility).
(I'm sorry if there are any mistakes in the text, it's just that English is not my native language. I'm also sorry if I made any logical mistakes, as I just started reading "Magnus Chase and the Gods of Asgard", which could have caused me to miss some nuances.)