[sfjailbird]

Not sure which sources you are referring, but the lettering looks like the earliest Islamic calligraphy (Kufic script, which is beautiful) [1].

This is the word Allah in Kufic:

https://freeislamiccalligraphy.com/?portfolio=allah-square-k...

The line above is ornamental. Pretty close to the clothes design.

[1] https://en.m.wikipedia.org/wiki/Kufic

[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.