End to end encryption implies that you have established an encrypted connection with a known participant and secured it by verifying your keys either off-band or via some CA provider to prevent MitM attack.
If your chat partner is anonymous, and your only connection is via offered service, there is no way to determine if you are being spoofed.
So anonymous end-to-end encryption is an oxymoron. I wish people would drop this e2ee fetish, insisting it everywhere, and would learn how it works, so they would be using the real thing where it makes sense, instead of opting for readily supplied snake oil.
In this case authentication is provided through a shared secret (the room name). An arbitrary MitM wouldn't know the room name since the connection to the server is also encrypted. Unfortunately this isn't much of a secret, since the server also knows the room name, but it's a start. There is an open issue to add password-protected rooms which would fix that problem.
In most e2ee threat models the biggest source of threat is the server that mediates the data exchange. Other threats are sufficiently thwarted with TLS, so ... ... these Briefing guys should better bring on these password protected rooms if they want to put this end to end encryption claim on their website.
End to end encryption implies that you have established an encrypted connection with a known participant and secured it by verifying your keys either off-band or via some CA provider to prevent MitM attack.
If your chat partner is anonymous, and your only connection is via offered service, there is no way to determine if you are being spoofed.
So anonymous end-to-end encryption is an oxymoron. I wish people would drop this e2ee fetish, insisting it everywhere, and would learn how it works, so they would be using the real thing where it makes sense, instead of opting for readily supplied snake oil.