Earlier quoted context omitted.
It backups your keys encrypted with a password only you know at server side (opt in) When you login, you provide that password, and Element can use that to decrypt the keys and use these to decrypt the messages stored in the cloud Alternatively, you can solely rely on syncing keys between verified devices.
That is not 'true e2ee' at all, and more of a security theater.
Only you as a recipient can decrypt the messages, because they are sent e2ee to you.
And this stays true if you use the backup, as only you know that password.