[Agrajag]

To me, the answer is simple, just look at the ghoul guardian description: "...In the ruined Keep, they still live and exercise their military code, ready to march if called by the Lord of Ulm. ..."
Exercising their military code would imply some sort of military structure, which would require commanders (that could lead them in battle). There's also the "...ready to march if called..." part, if they have no commanders of their own they clearly aren't ready to march anywhere.