Not necessarily, there are tons of things that are very secure but not at all private.

But… having a massive attack surface and known security issues and thinking your data is somehow private because you use a FOSS application is silly.

EDIT to answer your edit: partly. I mean regular timely security updates seems to be a struggle for most of these android OS but also because it provides a bunch of privacy by default options that these other android ROMs don’t such as a private relay, default RCS messaging, and makes them easy for users.