Sandboxed Google Play provides better security, privacy and usability. Why that is, as well as your question about Calyx vs Graphene, is answered in this Privacy Guides Blogpost.

conveniently ignores

Is this about how private and secure the actual usage of the actual OS is, or what?

Ok, that's fair:)

It's not the fault of GrapheneOS that there are so few phones with the necessary security. They can't reprogram your hardware.

More importantly, it doesn't change anything about Android > iOS.

Wearing special anti-surveillance clothing because you want to make sure cameras can't identify you is the real life equivalent to installing ten privacy browser extensions because you want to make sure websites can't track you.