I'll still bet good money that there have been high level TKD practioners who got involved with TKD after witnessing a demo. Maybe the demo was a flashy show designed to attract the attention of people who didn't know a lot about martial arts? Many martial arts give public demonstrations, partially to attract new students. That doesn't mean the content of the demo is what the student learns.
One example off the top of my head (although not a TKD example) is that Steven Seagal got involved in martial arts after watching an Aikido demo at the half time show in an American football game when he was a child. Regardless of what you think of Seagal as a person or an actor, the guy got to a very high level in martial arts (high rank in Aikido, taught a Japanese martial art in Japan). That all started by watching a demo.
"Let your food be your medicine, and your medicine be your food"