AppArmor is simply less granular. For example, it doesn't provide true RBAC or MLS security models. It also uses paths instead of inodes, so a hard link can be used to override some policies.

So it just depends on what the exploit or attack is trying to do. If an attacker gets root and is trying to overwrite a file, they may be able to. Maybe they can't, but they could probably still execute any code they can write and compile themselves. And perhaps they can write to other files and do damage.

SELinux and similar systems allow a lot more granularity. Programs and users can only talk to explicit what they are allowed to talk to, and maybe you want to limit the access to say, append instead of full write access.

It just allows a lot more granularity and restriction, that's the difference.