Sarcasm is an adjusted stress reaction.

there's a glossary at the end of the book, explaining Winston's vocabulary, and that this glossary was written in the future

Yeah, Atwood does a similar thing at the end of Handmaid's Tale -- altho her ending is much less awful -- with the symposium in the future talking about the book we've just read, and we know from just reading Offred's story that obviously she made it out and told someone. (Which is at odds with the very stream-of-consciousness day to day diaristic ((sp)) quality of the narrative, but anyway.) But yeah, like you say, there's no evidence in either Atwood or Orwell to show how civilization moved on from that nadir, so there's a kind of hollow quality to the reassurance.