To me, the major meaning of this statement is that the whole idea of this society is to control the minds of the people.
In order to understand this, let us think about the nature of Newspeak. The point of this language is to limit the ways in which people can think. If they have no words for various concepts, if the Party has told them what words they can use, the Party has limited what they can talk about and has thus limited how they can think. I think this is perhaps the major theme of the book -- the Party is trying to control the people all the way down to controlling their thoughts. By saying Newspeak and Ingsoc are synonymous, we are highlighting this fact.