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.
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.
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.
https://www.youtube.com/watch?v=boB6qu5dcCw