If only humans and Forerunners could interact with Forerunner tech, the Covenant would have never been able to reverse engineer Forerunner devices, or repurpose Forerunner machinery for their manufacturing purposes. The constructs that appear to be reserved for human and Forerunner use are military installations of extreme importance. However, in the case of Jul 'Mdama, it can be inferred that the Didact gave him authority over Requiem since he was given limited control of the Knights.