Can you expand on this? I'm willing to grant that it is true in practice, but it is not at all clear to me that a benevolent, lawful dictatorship impossible in theory.

A dictator can be benevolent, but he's out of the Law's reach, or he wouldn't be a dictator.

If everybody must follow the same laws, and nobody can change the laws at will, you will get a democracy. If some people can steer lawmaking at will or law does not apply to them, it doesn't matter if you vote on public representatives, they won't be submitted to your wills.