Why do you think that Squealer's argument was ‘unanswerable?'
I assume that you are talking about the part at the end of Chapter 5 where the animals want Snowball back. Squealer asks them if they really want Jones back. Orwell says this was unanswerable.
Snowball's argument is unanswerable because of course none of the animals want Jones and his oppressive ways back again. But the point is that Squealer is setting up this false choice for them. He is telling them that they have only two choices: 1) do everything that Napoleon says or 2) Jones will come back. This is false. There could have been other ways of doing things that would not be oppressive likes Jones's way or authoritarian like Napoleon's.
I think that Squealer's argument is unanswerable partly because the animals are not smart enough to see that he is offerring them a false choice. It is also unanswerable because it is a quick and memorable saying (we'd call it a sound bite now) that would have to be answered with a long (and maybe hard to understand) discussion of the idea of a false choice.