You know, I didn't try, they are animated in the background and not static but I'd be quite surprised if they let you murder the balloon riders lol. Time to take a sniper out there and try

So, after bringing in a sniper and getting to a high balcony with a vantage point of the closest balloon:

I can hit the balloon with the sniper, and it actually has a hit effect and I can hear the bullet impact the material, but after blowing through both mags that come with the rifle, the balloon did not go down.

I can also shoot the passengers of the balloon, who also have the same hit effect, but they are not bothered at all by taking sniper rounds to the face.

Myth busted