And you can add that parrots aren't as smart as people, and never needed a symbolic language in their natural environment so they never evolved the advanced wiring we did for this particular behavior. But anyone who knows parrots knows they're smart, they're social animals, and they do communicate vocally. Whether it's symbolic or not is debatable. Personally, having owned one, I'd say they do have some capacity to symbolize, but perhaps not as much as the article implies.

Perhaps they're as good as they are at it because the behavior and intelligence they do bring to relationshipos with humans is adaptable to our communications systems. Perhaps they have a collateral skill for communication, just like we have for music and mathematics and playing board games, behaviors which could never have evolved for natural reasons, but which we are nevertheless very good at.