Notes that sync across devices without an account

Really good question! No account doesn't mean no pairing: you'd link two devices once, by scanning a QR code or typing a short code shown on the first one. After that they find each other themselves, and the notes are encrypted with a key that only lives on your devices. Syncthing works roughly like this. The trade-off is that there's no "forgot password": you'd keep a recovery phrase somewhere safe.