Sure, but a person paddling isn't hydrofoiling, right? They're using their arms instead of legs and contending with tons of additional friction/drag. I see this as being akin to bicycling, since it uses the same muscles.
Counterintuitively, that probably makes it harder.
With any wing, the faster you go in level flight the less drag is caused by lift. This strange fact is because moving a large amount of fluid slowly is more efficient than moving a small amount of fluid fast, and a faster wing can interact with more fluid mass per second ("m dot").
Since skin drag increases with speed, adding these two drag curves together forms a 'valley' in the overall speed-vs-drag curve. Going slower or faster than this ideal speed will result in increased energy per mile.
The math is better explained in David MacKay's brilliant ebook, 'Sustainable Energy Without the Hot Air.'
They seem to be struggling a bit and the there's no wind or waves. Maybe coupled with a battery like an ebike it might be sustainable for longer than a few minutes.
Eh, it's hard to tell power output from pedaling cadence unless it's a fixed gear ratio. Most people pedal between 70-100 rpm regardless of the watts they're producing.
There are some clues in their body language. they appear to be straining. And the pedal movement looks a little jerky, as if the load is changing dynamically in relation to the effective flywheel/inertia of the system. Spinning a higher speeds with less torque is supposedly less tiring. So they might want to gear this down a bit and include a larger flywheel/inertia. Very impressive device tho. Maybe a hybrid approach with a solar panel for charging on the beach...
To me it doesn't look like it they were struggling with the physical exertion, but it does seem like they are struggling to properly hold on to the boat. It looks like the boat suffers from poor ergonomics, and needs some proper handles for holding on and steering.
