No, they did it because he said that he set the cruise control to 54 mph (a very specific number), which is clearly refuted by the data. In other words, it destroys his credibility.

Speed affects ICE cars range too, and at 80 it can be a significant effect.

Because of air drag which is proportional to the square of the velocity. At highway speeds, a large part of the energy consumed by the car to move at constant velocity is spent overcoming air drag. I think that's also why they are calling him out on going 60 mph on cruise control instead of stated 54 mph.

They did it to prove that he lied about the speed he was driving.

He claimed to be driving at one speed. He was driving not at that speed, and sometimes significantly faster.