Your dumb phone is much insecure than a smartphone which has GrapheneOS or LineageOS. Your dumb phone certainly lacks secure 4G or 5G communication, cannot use e2e encrypted messaging platforms and cannot update firmware in case of security bugs.

Did you even read the article? 0-day vulnerabilities are not unique to WhatsApp. Even Signal can be exploited given it’s complexity.

SimpleX is not a Signal fork. It is it’s own protocol, service and app. It just utilizes Signal protocol for encryption like every good e2e encrypted messenger out there.

SimpleX allows anonymous identity, federation between servers and still a good UX.

This is why I like GrapheneOS on phone. It is hardened and secure, but never gets in the way of your work. Everything works as it should. Kicksecure is the closest on the desktop space, though Fedora is also reasonably secure.