Sanderson intentionally did not tell his cousin about overhearing someone say that the factory would close, knowing that if he withheld this information, his cousin would assume it would remain open. Clearly this was morally wrong. After all, lying is morally wrong. And making a statement with the intention of misleading someone is lying. True, it was Sanderson' s failing to state something that misled his cousin. Yet there is no moral difference between stating and failing to state if they are done with the same intention.
Sanderson believed that the factory would in fact be closing is an assumption required by the argument.