"The peer-to-peer structure really protects them from sudden scale in demand."

In theory, for sure. But in reality the p2p part might not work for lots of users for various reasons (firewall etc.) so then the server gets all the load and chokes. At least that has be my experience when experimenting with webRTC.

"Last year, thanks to French indie journalist David Dufresne’s Au Poste ! livestream show and his hoster Octopuce, we got a livestream stress test with more than 400 simultaneous viewers "

And 400 is not a very big number, if one has ambitious goals. But it is a good start.