So yeah, as root privileges still exist and are accessible, they still allow you to modify the system, but you're not *supposed* to. By default, even root sees /gnu/store as immutable, but it can change that on its own.
In contrast, Fedora Silverblue sets the whole filesystem as read-only and mounts all stateful paths, either from modifiable partitions or virtual filesystems, into /var subdirectories. Reading this article though, I presume Silverblue is just as vulnerable once you get root.
https://www.redhat.com/sysadmin/immutability-silverblue
I've also tried Fedora Silverblue, which is neat because it's immutable, aka the OS itself can't be easily changed and made vulnerable or unstable