In spite of Napoleon attempting to blame the destruction of the first windmill, we are told that the humans believe it occurred because the animals did not build the walls thick enough. Thus, although the "myth" of Napoleon's treachery continues and his supposed destruction of the windmill is held as proof of his treachery, the animals are instructed to build the windmill with thicker walls to prevent any other acts of "treachery" occurring to destroy the windmill. When they finish the second windmill in Chapter 8, they are understandably proud of themselves:
Tired out but proud, the animals walked round and round their masterpiece, which appeared even more beautiful in their eyes than when it had been built the first time. Moreover the walls were twice as thick as before. Nothing short of explosives would lay them low this time!
Of course, this last sentence is ironic because that is precisely how the walls will be destroyed the second time the poor animals have to face the destruction of all their labour and toil.