This is such a great video... and I believe him because I used to be able to ride a bike from about the age of 4-6. I didn't ride for 2 years... and when I was about 8 and tried to ride again... I couldn't. I had to learn all over again. So I know it is possible to forget how to ride a bike!
I know what this guy is doing is totally different from my experience (and totally cool), but I can believe that the brain can change like this!