Actually he was not ballistic. He was the opposite. The picture below is a missile under power. The rocket fuel is burning.
The next picture (below) is of a missile in its ballistic trajectory: after the main rocket motor has fully burned. Guidance motors still operate, but the missile is not under power, it is slowing down although it is traveling very fast. From the time the main motor fully burns, the speed of the rocket is slowing and is determined by momentum, gravity, and wind resistance.
A missile is ballistic when it's flying toward its target after all the propellant has burned. It is coasting.
My friend was describing quite the opposite. The fast rider was under power and had more power than any other rider in the race. If he were actually "ballistic" he would be coasting.
So a science/technology word enters popular culture meaning the opposite of its actual meaning, which is
1. of or relating to projectiles or their flight 2. moving under the force of gravity only.
Of course, the most well-known opposite word is Theory. In science, a theory is the BEST possible summary description of a body of research. So the Theory of Universal Gravitation describes the motion of all objects including missiles, both under power and in ballistic flight.
But in Pop Culture, "My Theory" describes nothing more than an opinion with no necessary connection to fact or reality. My Theory could cover things like believing fluoride is a communist plot or that Stanley Kubrik fakes the moon landing.
Without "Going Ballistic" "My Theory" is that most people don't care much about the words they use.