A Czar can lay down his power, and so also can a Parliament.

To argue or imply that because sovereignty is not limitable (which is true) it cannot be surrendered (which is palpably untrue) involves the confusion of two distinct ideas.
It is like arguing that because no man can while he lives give up, do what he will, his freedom of volition, so no man can commit suicide.

A sovereign power can divest itself of authority in two ways.

It may put an end to its own existence or abdicate.

It may transfer sovereign authority to another person, or body of persons, of which body it may, or may not, form part.