I would think that at this point, Nguyen himself is now a naturalized citizen, not just an immigrant. Yes, the letter could have been a little more clear and could have said that legal immigrants, unless they are naturalized citizens, are not allowed to vote.
Personally, I don't see anything wrong with informing people of what the law regarding voting is. I would definitely not consider this a hate crime.
Think about it as a poor translation from Spanish to English ~