There are two huge flaws in their logic: The first is that using a plural pronoun for a singular meaning is nothing new for English speakers. A few hundred years ago, the second-person you was exclusively a plural; thou was the singular version (e.g., “Thou shalt not kill,” “Thou shalt not lie”). Eventually, you extended to the singular meaning and pushed out thou entirely. Who’s to say the same thing couldn’t happen with they?
