I've run over 70 mph a few times but I run mostly around the same speed as you.
However the guy you saw may have pulled that setup many thousands of miles and knows exactly how it will react, so he feels perfectly fine going that fast. Just does no good trying to second guess the other driver and will drive you crazy. :) If he wasn't weaving or moving around I'd just say c'est la vie and let it go.
I know some people that feel driving over 100 mph on a bike is just nuts, but mine feels perfectly stable up to, and over 130 mph. To-may-to, to-mah-to.