They actually also have an onion service: https://proton.me/tor
They should probably use the Onion-Location
extension http header though, imo.
When I accessed their front page through tor,it did not auto-redirect to onion >.<, even though I have that setting enabled.
That’s one of the advantages with p2p-ish networks, if you’re on the same network, devices can often disover each other and communicate directly, bypassing the bottleneck of an internet connection like traditional cloud services have .