I'm don't think it's possible to start a bike by pedalling with zero forward momentum. You will fall over. You need to kick off - start pedalling with the bike already moving forward. So you're right, and a third neuron is certainly not sufficient. You need legs, too, and arms, and a torso, and motor neurons, and respiration/metabolism. Clearly, the paper has no practical application; if you need to ride a bike, it's far cheaper to hire a human to do it.

If you orient the pedal so it's at about 9 o'clock, and hop on, the downforce will produce sufficient forward motion for steerage. Of course you can also push off with your other foot, but it's not essential. But that's another neuron. ;-)

You absolutely can start a bike by pedaling with no forward momentum. You can see it when someone starts pedaling again after a track stand.

https://www.youtube.com/shorts/zwBW2Akw1P0

Possible, but more complex than most appreciate. Pushing a pedal down shifts the rider's center of gravity over that pedal, requiring the bike to lean in the opposite direction to maintain a straight line. This done by the rider counter-steering to command that angle. Watch in the video how the front wheel and lean angle alternates left-right in time with the pedal pushes. Once at speed, the gyroscopic forces of the front wheel mean the rider doesn't need as much lean angle, so the wobbles get less and less. Compare an accelerating motorcycle where the rider doesn't shift weight and therefore doesn't need to wobble the front tire.