RE: It is certainly not illegal for immigrants to vote.
Bad translation. The Spanish word for a resident alien is similar to the word immigrant. The word is imigrado, meaning, one who has left one's country of citizenship and moved to another country. It means a green card or other visa holder. In Spanish, immigrant would be imigrante, not imigrado.
That's my impression as well. I don't think there is any intent to mislead in the letter.