It is hard to convince oneself that this is substantially different from a glider.
Neither is a human powered ultra light weight, huge aircraft like the gossamer albatross. If you have a very weak engine it needs to be made out of incredibly light materials, and even so light it must be flown indoors with zero wind.
I didn't happen. Look at the video they link. All the forward momentum and lift comes from the launch rail. It simply glides to the ground after that.
2:56 certainly looks like sustained powered flight to me.
I would be more inclined to believe it if it didn't fly out of frame before it had even been in the air as long as the "unsustained" flight. I simply can't imagine why they are so careful to obscure their sustained flight from scrutiny. I think it is healthy to have a measure of scepticism.
They noted that the first successful test was at 50% and effectively a power glide. Further in the video you can see they do achieve a sustained flight that reaches the end of the gym and is more than just a glide.