You're right, of course. I must've misread originally. I tested this in my own setup and you're correct, "USM Group Modification" ACL item doesn't seem to apply to Account contacts. As documentation states, not all ACL options apply to Account contacts, wonder if this is by design then. You could get around it by creating a normal user and linking it to the ACL, that works properly.
It would be weird though if it is was by design, as UMP users are supposed to be created under in the contacts (if they have no access to other clients)
-jon