You mean a security model that misses the fact that the kernel cannot be updated and relies on a "sanboxing" solution that doesn't bother limiting kernel attack surface. No, I don't think they even had a security model in mind, a threat model or anything beyond random ad hoc ideas. And if they did some thinking, they would not have chosen SELinux either, as it's not a decent solution to anything, it's more like a solution to "we must to do something, this is something, we must do this".