Pretty obvious, I would have said.
While available evidence suggests that the events of September 11th negatively influenced the relative earnings of employees with Arab background in the US, it is not clear that they had similar effects in other countries. Our study for Germany provides evidence that the events also affected the relative earnings of Muslims outside the US. However, the results show that there was no uniform effect on all types of Muslims across all types of firms. Accounting for moderating factors, a significantly negative effect can primarily be found for low-skilled Muslims employed in small- and medium-sized firms. This conforms to theoretical expectations. Moreover, we demonstrate that defining appropriate treatment and control groups is crucial for identifying the effects.
But I think the article confuses Arabs with Muslims. All Muslims are not Arabs and all Arabs are not Muslims.