Placed in the perspective of the current political climate in Sweden I can not be but sceptic against research which tries to make islam part of Sweden's history. While the article is published by the BBC, the research - for what it is - was done in Uppsala.

That Vikings travelled to islamic countries is known. That they traded with them - willingly or unwillingly - is also known. They did the same with many other cultures and brought materials back to Scandinavia. There have been finds of Buddhist statues, swastikas, Christian ornaments from the time when the Vikings were still going the way of Odin and Thor, etc. There is, however, no research showing the Vikings were making Buddhism part of their culture even though they used some of their symbology (swastikas come to mind). Why not? Because there is no political drive to give Buddhism a part in Sweden's history while there is a strong drive to do this to islam, especially in parts of academia.

Also, if these Vikings wanted to show allegiance to allah and Ali, why did they do so in mirror image?

This belongs to the realm of Beatles covers showing mirror images of Paul McCartney being dead, backward satanic messages on rock albums and chemtrails: the conclusion was the starting point, all that was left was a search for 'evidence'.