I would imagine that power to weight ratio is more important than just the power. Also the smaller the cyclist, the smaller they can build the aerodynamic shell, thus reducing drag.
It then follows that bigger guys do well pushing against air (since frontal area grows slower than maximum aerodynamic capability), and smaller guys do better dragging themselves uphill (since mass grows faster aerodynamic capability). You can easily find practical examples of this: say Fabian Cancellara, who has won time trial world championships 3 times + almost anything else riding in fairly flat ground, including several tour prologues and stages before getting into mountains... but when you get to the mountains, it's the featherweights that rule, so likes of Cancellara can't ever win the tour.
So... to propel something as fast as possible against air, you need big and strong cyclist (with huge lungs to match), just like the guy in the video looks like. Power/weight has little to do with it, since air drag is most of the resistance, and they can accelerate for 5 miles before measuring speed (you would need power/weight if you needed to accelerate fast, which is not the case).
Because frontal area doesn't increase proportionally with weight, I would argue that power output should be the primary concern and that typically increases substantially with weight. For evidence of this, look at world class time trialists versus world class hill specialists.
Fabian Cancellara [1] is considered one of the top time trialists in the world at 181 lbs as he can put out much more absolute consistent power than someone like Nairo Quintana who is considered one of the world class climbers and weighs 128 lbs, [2] even though Nairo Quintana may have a better power:weight ratio.
[1] https://en.wikipedia.org/wiki/Fabian_Cancellara [2] https://en.wikipedia.org/wiki/Nairo_Quintana
https://en.wikipedia.org/wiki/Tony_Martin_(cyclist) weighs in at 75 kg - lighter than Cancellara - but not that light for a professional cyclist either.
Would be interested in knowing, if a human powered vehicle on steel rails with steel wheels with the same air resistance could be faster than this just due to the lower rolling resistance.
Weight shouldn't have as much of an impact on acceleration because as muscle mass & power goes up, weight goes up as well.
But more weight inside the shell doesn't increase air resistance, while it does increase power. So a heavier rider with greater sustained power output will, in an environment where wheel friction is trivial, go to a considerably faster top speed on flat ground.
If they're putting athletes into this thing, the bigger the athlete, the more force (okay, the more power) is available for propulsion. They have bigger muscles which store more glycogen and have greater total mitochondrial activity, because there are simply more of these cells available to do the work.