Yeah.... The bike and wheel they show only has one gear. Look at how the people are pedaling. Or trying to. They're going faster than they can pedal.
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And it's using coasting brakes, or whatever they're called.
The idea of swapping out a wheel is interesting. I'd want to keep all my stuff the same though, just swap out a wheel, have everything work, and have something hand-operated to control the extra power. Those are available, but... I also see posts about all the extra wear-and-tear. The spokes are going to get a lot of stress.