What are the estates for? Not mainly so that the state gets the best advice. They only add something to that.

Their real point:

The citizens who have no share in government should share in knowing, deliberating and deciding. In this way their freedom gets its right.

What follows from this?

The proceedings of the estates must be public. Then everyone can know about them.

Summary: The estates give citizens a share in knowledge, deliberation and decision. So their proceedings must be public.