Notice while it is picking up speed it is slower than the wind.
How could it not be when starting from a full stop? What you're suggesting would be fraud, and if you read back you will find that it isn't the case here. Common sense mandates the speed limit, regardless of "Sail" size should be very close to the speed of the wind itself... and without the gearing this would be true. What they've done (quite ingeniously) is invented a way to harness the additional "sail" power that would otherwise be wasted.
Think of it like this: If 10 square feet of "sail" were sufficient to power a craft to nearly windspeed; of what use would an additional 10 square feet of "sail" be? (redundant waste)
You could apply X-amount of braking resistance (approximately one half of the wind's captured power's worth) to such a craft with double the "sail" with no noticeable decrease in speed, right? Rather than using braking resistance; you could build a gearbox that used that same resistance to turn a prop that blows back into the wind itself, couldn't you? This is essentially what they've done.
The craft doesn't store the excess energy; it converts it to opposing "wind."