It's the usual "half a story" - The half they WANTED to tell!!!
Firstly, as has been said, the car they used makes all the difference. A small-engined boxy-shaped car will be MUCH worse as it gets closer to the throttle being wide open. It would have been good if they'd said what car it was, or better still, tried it on a number of different cars. Of course, it wouldn't look good when they found that a big Jag didn't use appreciably more fuel at 80 than at 70!
Secondly, the old "aircon swindle"! Let's assume that the air temperature was 20 degrees C. Let's assume also that the aircon had been OFF and that the car had climate control (because that illustrates the point best) and the climate control was set to maintain a cabin temperature of 18 degrees...
The instant the aircon is switched on, the pump will run at full capacity (OK, there are different designs of pump these days but I'm talking about the most common fixed displacement type). Now that WILL use a fair bit of power. As a percentage, of course, it rather depends on the power of the engine in the car!
What happens next is that the evaporator temperature drops and the air getting sucked through it and into the car cools down. If the difference in temperature between outside and inside is very great, the pump will probably never stop running. If not, the evaporator will get progressively cooler until the anti-icing switch cuts the compressor off. AT this point, it will use ZERO extra fuel until the evaporator has warmed up again to the point where the pump cuts back in. And this goes on. The bigger the difference between the inside and outside temperature, the more of its time the pump will spend running.
So I think what happened in this case is that the pump cut in initially and the bloke with the laptop took his reading. If they'd waited a couple of minutes, I wouldn't mind betting the reading would have been different (and smaller)!
So basically, when the aircon is first switched on, it uses a fair bit of power and then, depending on ambient temperature, it cuts out and once the evaporator is noce and cold, it won't use anything like as much power to keep things cool.