It's something closer to Tailscale DERP.

We evaluated QUIC (and many other approaches). Turns out it's a lot harder than you might think to move traffic at high speed across the world, over residential-grade internet, and not drain your battery.

Did you consider Iroh and if you chose not to use it, why not?

If it's truly p2p, some relay would be there in case the client cannot be reached through NAT. Not sure how they would bear the cost of the bandwidth for unlimited transfers in that case

Traffic is pretty cheap outside the big clouds. For example Hetzner charges $1/TB on a 10Gbit connection

Their 1Gbit connections are unmetered.

I wouldn't be surprised if it's built with Iroh

https://www.iroh.computer/