Encryption is different. If you encrypt data with a generated password and then delete it, you're toast, and passkeys are no different. I think the author is arguing that users may not even realize that the passkey itself is needed to decrypt, possibly because they're so associated with login.
Passkeys are effectively just long passwords you cannot see. The mechanism is just gravy.
Sites usually have the user SEND their password to the site to authenticate. There is no need for sites to be written that way, but that is how they are written.
Passkeys cannot, by design, be sent to the site. Instead they use a challenge-response protocol.
You can remember a strong generated password if it's a pass phrase. Better "rememberability" with the same amount of entropy.