Does Android's implementation of SELinux's MAC inhibit apps with root privileges from accessing other app's data?
For future visitors' reference, it depends on the SELinux context of the root process. In DAC UID 0 is the super user which can access any resource on the device allowed by kernel. MAC (SELinux in our case) is denied by-default i.e. no process can access anything unless a policy rule is defined to allow the access.

So there is no super context in SELinux but it's possible to define one. That's what rooting apps do since Lollipop. Magisk, for instance, defines u:r:magisk:s0 with unrestricted access to any other context. Any process running with UID 0 and u:r:magisk:s0 would be able to access everything on device including other apps' data.

It depends on the exploit with what context it's able to run a process. With SELinux enforced, it's very hard to inject something to policy.

For details see:


What sepolicy context will allow any other context to access it?
How Magisk works?