Why is Squealer's argument in chapter 5 of Animal Farm considered "unanswerable"?

Chapter 5 sees the expulsion of Snowball and Napoleon's rise to military ruler instead of co-equal leader. When the other animals question Napoleon's motives, Squealer explains that Napoleon has known for some time that Snowball is a traitor to the farm, that he has been selling secrets to the farmers, and that he plotted to kill Napoleon and take control (just like Napoleon actually tried to do). He finishes with his classic appeal to fear:

"One false step, and our enemies would be upon us. Surely, comrades, you do not want Jones back?"

Once again this argument was unanswerable. Certainly the animals did not want Jones back... Boxer, who had now had time to think things over, voiced the general feeling by saying: "If Comrade Napoleon says it, it must be right."
(Orwell, Animal Farm, george-orwell.org)

The argument is "unanswerable" because of the implicit notion that arguing with Napoleon will bring Jones back to the farm. This is not true, but because Squealer repeats the argument so often, the animals have begun to believe it. By refusing to think through the problems and see how Napoleon is actually doing what he accuses Snowball of plotting, the animals prevent themselves from coming to a sane answer for Squealer's argument; essentially, it is unanswerable because they literally cannot see past it, preferring, as Boxer does, to have blind faith in Napoleon.