Not OP, but video nerd. I haven't actually used the product yet, but my guess is they're using WebRTC, which can be run (almost) completely as a peer-to-peer mesh network for small enough groups.

Where video can start getting really expensive is, as you said, when you're centrally hosting/processing/streaming large amounts of it. For this use-case there's a good chance they don't end up touching the video streams themselves at all, which is really cool.