Actually, thinking about this while heading in to work this morning, I'd like to revise my criticism.
For much of riding a bike we rely on instinct. When I ride a reverse steer bike, I have to think about what I am doing (and do the opposite of what instinct tells me). If I try to ride a reverse steer bike too fast I fail because I don't have enough time to think and instinctive reactions take over.
In the video, he may mean that he rode only a reverse steer bike for 8 months and riding it became instinctive. If that was the case, then riding a normal bike would not be as easy, as he would need to think about what he was doing - I'm still surprised it was as difficult as he made out, but I think the video was somewhat dramatised and was all about making a theoretical point.
Many skills can be learnt to the point where they become instinctive, and riding a bike is probably the most common, thus where the idiom about forgetting how to ride comes from.