English isn't my native tongue so perhaps i'm missing some subtlety, i thought ethnic groups are people who share a common descent?

Honestly asking, not trying to nitpick.

You're correct. The point is that for NA that common descent is Arab not African. I know that's not completely true but in the context of the crude racial classification system the US uses it is. According to the US government you're white:

>White. A person having origins in any of the original peoples of Europe, the Middle East, or North Africa. It includes people who indicate their race as "White" or report entries such as Irish, German, Italian, Lebanese, Arab, Moroccan, or Caucasian.

https://www.census.gov/quickfacts/fact/note/US/RHI625219#:~:....

Ethnic groups are a distinct concept from racial groups. Ethnicity has more to do with culture and society versus race, which is largely about appearance and, to an extent, geographical provenance.