I think you have to decide to be able to, before you sit on it. It's a really interesting example though. Some people would watch others doing it, go "nope, too much effort, not worth the trouble". Some might try it and give up. Very few would take the time to conquer it like this man did. But he did, and you can see it was becoming second nature. It sort of shows that the brain can adapt to change it's natural reactions (which I think is sort of what sexual orientation is). But what if this became something so contentious as sex, and you have some who are staunch believers in the normal bike and some who believe it is their right to ride a backwards bike if they wanted? Would that lead to each party saying they were born that way and wouldn't change no matter what? Good point about child-brain elasticity. I wonder whether children's experience in life is so little they rely on grown ups to teach them stuff, they are accustomed to being corrected (learning) and is not until later when they begin developing a sense of identity and rights, that they begin asserting their point of view and that attitude gets more or less lost.
Also think about this, as I am good at reversing a normal car trailer. But I have only once tried to reverse a trailer with a turntable front axle. That was hard and I think I couldn't. I forgot when actually. Truck drivers can do it though, and they can turn it on and off. Also, it's a natural skill now, but I suppose about three or four times my habit has failed, and I had to stop and think about it and snap back into that trailer steering mode where steering left makes the trailer go right.
So this guy should be able to snap back and forth with practice just like truck drivers can snap back and forward between trailer types.
To illustrate the point I am making here, yes we need to decide that we will do it before we begin doing it. Likewise, I reckon we can expect, if it was normal that all bicycles had this backward steering, people would struggle to adopt straight-forward steering. But as I mentioned, our ability to adapt is further hampered by beliefs that prevent us from even wanting to become comfortable with a new habit.