I actually thought that was debunked years ago. Like back in the 80’s? And that it was determined you were not saving any gas by not using the AC and rolling down the windows.
I have a feeling those studies were done at highway speeds, by running a car with the a/c on, and then running it with the a/c off, and all of the windows down as far as they would go.
In local stop and go driving, running the a/c is definitely going to use more fuel than rolling down the windows. And especially if you're accelerating. Once you're out on the highway, loafing along at a constant speed, the a/c probably isn't affecting you much, but at the same time, whenever I've been rolling along at 70 mph or whatever, I'm not rolling the windows down as far as they'll go. They're usually only cracked an inch or so, at the most.
Back in 1978, GM tried an experiment with their downsized intermediates (Malibu, LeMans, etc). By that point, we were pretty much accustomed to the rear windows of coupes being stationary. Well, for '78, GM made the back windows in the 4-door models (sedan and wagon) stationary as well, and you only had a flip-out vent window. In the sedans, it was in the C-pillar, but in wagons it was in the back part of the door. In their wind tunnel testing, they determined that you got better airflow when you rolled down the front windows and opened the vents in the back, than you would have if they made the back window roll down. The air flowed through more effectively, whereas with a roll-down window in back there was a more chaotic turbulence that messed with the aerodynamics.
This was fine and dandy, at highway speeds, but again, in local stop and go driving, the back seat passengers would bake, and it made air conditioning almost mandatory. And that was back in the day when even in such lofty cars as an Electra, Ninety-Eight, and New Yorker, a/c was still an option. You had to go with a Cadillac or Lincoln if you wanted it standard.
People griped about those stationary door windows, and even today, in classic car forums, if someone mentions one of these cars, people come out of the woodwork to complain about those stationary windows. But, the cars continued to sell well, in their day, and GM saw no need to modify them, right up until the final Cutlass Supreme sedan rolled off the assembly line in 1987.
Anyway, one test you can do, yourself, is to get out on the highway, and then put on the a/c. See if you notice a drag on the engine's performance. Then, try it with the a/c off, and the windows all rolled down as far as they'll go, and see if you notice.
The a/c is also going to hurt your fuel economy more, with smaller engines, than a bigger engine that just loafs along.