In this quote, Orwell uses a language technique called indirect quotation. The story is not repeating exactly what Squealer said to the other animals but simply summarizing the highlights of it, which is exactly what indirect quotation does.

We know enough about the animals at this point to realize that this must have been a much longer conversation. First, many of the animals are not very intelligent, and it can take many repeated explanations for them to grasp even a simple concept. Second, we know that Squealer, a skillful propagandist, likes to talk.

By providing the highlights of the conversation through indirect quotation, Orwell does not interrupt the flow of his story or bog the reader down with too much information. Through this technique, he keeps the tale moving at its usual rapid and compact pace.

This is an example of double-talk, or propaganda. Napoleon, who had been opposed to the windwill, now asserts through a clever lie that the windmill had been his idea all along, an inversion of history that is politically expedient. In fact, what the quote shows and that, history, and even truth itself, can be made into any shape the ruling class, in this case Napoleon, wants it to be.  As this eNote puts it:

What is most demoniacally human about the pigs is their use of language not only to manipulate the immediate behavior of the animals through propaganda, emotive language, and meaningless doubletalk but also to manipulate history, and thus challenge the nature of actuality.

The  concepts of "truth" and "falsehood" become empty signifiers in this kind of speech, in that the final determiner of what is "true" or "false" is not reality or experience but the state.