Good question. I would think I would see their username on this list https://grs.suprnova.cc/index.php?page=statistics&action=pool unless they only mined as anonymous for a short amount of time. The last found block on grs.suprnova.cc was October 2nd, so I would think with that hash rate, they would of found a block already. On the Groestlcoin difficulty chart https://www.coinwarz.com/difficulty-charts/groestlcoin-difficulty-chart you can see some major activity on October 3, the date the YouTube video was posted. They want 0.1 BTC to load the software. Kind of hard to justify that amount of money just to see if it's real. I think it would give me more proof if they showed their mining username on https://grs.suprnova.cc/index.php?page=statistics&action=pool with their hashrate.

... and I checked their BTC address and they have zero transactions, so nobody has paid them their fee.