The reader knows that the pigs realize they made a mistake in building the windmill’s walls too thin because when they rebuild it they make the walls thicker.
The humans do not believe that Snowball vandalized the windmill. They suggest the windmill was poorly designed, and would never work. The pigs disagree, but they decide to build the walls thicker.
Still, it had been decided to build the walls three feet thick this time instead of eighteen inches as before, which meant collecting much larger quantities of stone. (ch 7)
The building of the windmill is a great diversion for the animals on the farm. It keeps them busy and not asking many questions. Its destruction is a perfect chance to scapegoat Snowball. Yet it is clear that the pigs realize their mistake, because when they rebuild the windmill they do so with thicker walls. They just don’t admit it.