Atomic distributions don't provide the security of verified boot or the far better mandatory app sandbox and permission model on Android. Flatpak is catching up to Android from 15 years ago, but it isn't mandatory. Android continues making backwards incompatible privacy and security improvements via new target SDK versions on a yearly basis with apps forced to adopt it within around a year. It continues moving towards a more private and secure model for apps.
GrapheneOS makes our own major improvements to the permission model, but we cannot enforce apps adopting it and need to design it all to be backwards compatible. We do that for Contact Scopes, Storage Scopes, our Sensors toggle and other features but not everything can be done that way. For our exploit protections, we have toggles to work around apps with memory corruption bugs or bad practices such as dynamic code loading. Android can enforce apps improving in a way we can't do. We're starting from a platform with a mandatory app sandbox and relatively modern exploit protections though. Android also does support using hardware memory tagging or HWASan to test apps even though many developers aren't using those to clean up their memory corruption bugs.
Thank you for actively commenting in this thread with detailed responses