ReadID does not include an indication of citizenship and non-citizens can get them.
It is unfortunate that some of the items that can be used to get a RealID are also proof of citizenship and thus they could have added this information to the card at that time. I'm not sure what to do if somebody thinks they are a citizen but lack any of the acceptable proofs, they may have to get the RealID without the citizenship indicator if they need it soon, and there will have to have another option than a RealID to register to vote.
None of this has anything to do with using RealID or any other license or card at the polling station. That is ONLY to prove you are using the right name, you have to be registered in order to vote.