I believe it technically is, in a sense that Sharia (which is the supreme law) is supposed to be applicable even to the royal family. But I doubt that's how it works in practice.
The other side to rule of law is that the government can't punish you arbitrarily - they need to have an applicable law, and they can only apply it to you as written (including penalties etc). In a system without rule of law, a subject can be charged with wrongdoing on the whim of the ruler. That part is also missing in KSA.