Don't forget that you have to assume they were using some sort of special Tardis textile that's bigger on the inside. That, to me, is the most compelling criticism. Everything else is arguably circumstantial, but it's kind of hard to look past a hypothesis apparently resting on a physical impossibility.

It's hard for me to see this as a big moral loss, either. A more compelling poster child for Vikings' relationship with Islam would be the examples of Vikings converting to Islam that are documented in the historical record. The garment, even if you accept for the sake of argument that it does read Allah, could still just be a prized souvenir.