I think he talking about e-mails to and from proton mail and other email providers are not really encrypted. Only mails between proton mail are encrypted.
Individual emails are only encrypted if the sender encrypts them, yes, and it's only enabled by default between protonmail users. However, the mailbox as a whole is encrypted at rest with some one-way method that doesn't allow proton to decrypt it: https://proton.me/blog/zero-access-encryption
> However, the mailbox as a whole is encrypted at rest
The server still receives from other servers and from the client (if the email is not going to a proton account) the plain text, so nothing prevents them from making a copy.
They only have to comply with Swiss courts, but they're pretty up front about what they can and can't do, including suggesting Tor for anonymity, and the difference between Mail and VPN in Switzerland (VPN is no logs).