"send end-to-end encrypted (E2EE) emails to anyone, even if the recipient uses a different email provider" but the video shows Gmail asking the recipient to authenticate. How does that work? If a Gmail user sends an email to my self-hosted server, there is nowhere to authenticate me to.

And it means either Gmail or the actual email stores decryption keys, so what is the threat model in which E2EE is useful here?

The only "advantage" I see is that now recipients must manually archive these "encrypted" emails if they want to keep access to them in the future (so most of them won't). That would be consistent with Google's strategy with AMP's editable emails.