I am also not a lawyer, but asking if someone is an EU citizen is probably not acceptable in and of itself.
The DMA Chapter 1 Article 1 section 2 makes it clear that the requirement applies to services offered to users “established or located” in the EU.
You can be located in the EU without a residence permit or citizenship perfectly legally (and it’s not even clear whether being LEGALLY located or established in the EU is a requirement).
