They are a different ethnicity (ethnic?) and are counted as an Indo-European ethnic (ethnicity?).
It was just an example, in general when we use "ethnic" and when "ethnicity"? I checked "ethnic" in Longman, it could be both adjective and noun, and it confused me more about their difference:
Someone who comes from a group of people who are a different race, religion etc or who have a different background from most other people in that country
- In that neighborhood of New York, we were all ethnics.