a king is sovereign and thus cannot in his own body be subject to the law; he has subjects, is not a subject. in a republic or democracy, the people are sovereign, and thus no one is above the law, all are subject to it. calling him a king is an effective shorthand for where the SC put him.
also, plenty of kings, going way back into history, were sharply limited in their powers? the definition of kingship has rarely been "can do anything they want."
the point is that the king is in theory above any other form of law, and the law flows out of him. that's a rough way to describe the relationship roberts posits between the executive and his duty, that if the president does it, it is legal no matter what "it" is.
re: the limitations, I would say that the difference (in theory) was that the limitations are regarded as self-imposed by the king, from whom all sovereignty flows. in reality, those limitations were a function of a king not REALLY being all-powerful but needing to build consent of the governed
