Occam's razor:
Viking raided silk traders travelling far, or raided people who had access to those traders, traders who were either wearing or selling the silk cloth with pattern.
Viking raided silk traders travelling far, or raided people who had access to those traders, traders who were either wearing or selling the silk cloth with pattern.
You have to look at the garments in the mirror in order to see the word Allah so it could just as well be random patterns, like that piece of toast with Jesus' face on it.
It would not be the first time that the word Allah appeared among otherwise random patterns: https://en.wikipedia.org/wiki/Perceptions_of_religious_image...
As to why make the assumption, unless there's evidence of Scandinavian goods in Byzantium it's a good assumption.