Actually I've never had an issue with wind on the balcony cabins on the stern of the ship (aka, the aft-facing balconies). In fact, they are usually the most usable balconies for the very reason of not being buffeted by wind....which can often affect balconies along the sides of the vessel. The wind does not whip around the back in my experience. It is usually quite calm. Just go stand on the back of the ship on another deck and see how the winds are. It is no different for the balcony cabins on the decks below you.
One thing that can be a small problem is ash from the stack coming down during the night (usually the time they blow out the soot from the stack) and being on the balcony and furniture the following morning. But it has never bothered me that much. I prefer the stern balcony cabins!
In my personal opinion, the person from Carnival who talked to you doesn't know what they are talking about.