K8y does Graphene do anything more to keep apps separate from each other?
Yes. First of all, GrapheneOS tightens the restrictions against apps running on it, secondly it gives you more granular control for example with the Network permission, Sensors permission, Storage Scopes and Contact Scopes, and thirdly apps are always allowed to communicate with each other, each app decides which other apps can communicate with it and how, but GrapheneOS lets you more easily utilize different profiles (user accounts, Private Space, etc.) which prevent apps from communicating with each other even with the ways the apps themselves permit for other apps to communicate with them. Also GrapheneOS fixes some holes in communication of apps between different profiles. In addition, both GrapheneOS, plain Android, and iOS sandbox the internal components of the operating system behind the scenes, but on GrapheneOS this sandboxing is also strengthened, not just the sandboxing of user-installed apps.
Note that I'm not part of the GrapheneOS project, I'm just saying what I've learned from the GrapheneOS website. You can also check the features list. For example, there's a component called SELinux which is also used on Android to sandbox internal OS components and user-installed apps, and on GrapheneOS this SELinux component is used much better than on plain Android.