[mumblemumble]

Specifically the square version of Kufic, which is apparently a more recent variant.

First, the style of Arabic that Larsson says she has identified—square Kufic—is not known to have been used in the 10th century; it only became common about 500 years later.

I imagine it's just like how, with the Latin script, anachronistic letterforms can be used as a line of evidence in identifying less-skillful forgeries.