I'm really curious on this as well. I looked up "Frequency Festival in Austria" and it did come back with one but nothing anywhere mentions anything about people getting naked and singing together, lol.
Just a traditional music festival.
That guy who is the owner of the vimeo channel (Gerrit Starczewski) is a professional photographer. He has done several group shootings aimed at a younger crowd of participants, therefore he has placed his shootings as side events at music festivals. The name of the project was Naked Heart. He has also organised naked football (US: soccer) games. The results are on his vimeo channel, his website seem to been down.
I have read that he got into trouble because of a female participant of his shootings who sued him for taking pictures of her nude and publish them. That girl was underage at the time of the shooting. She didn't get a compensation, but he was forbidden to publish pictures with the girl. The trial was in 2017, it seems that he gave up his nude events after that.