Overall: I loved the series and felt satisfied by the finale even if it didn't hook me emotionally as much as I wanted.
I too think she was making it up. It's a perfect construction of a way for Lindelof to "have his cake and eat it too" regarding answering the one question he said the show would never answer -- where people went after the Departure -- to silence any potential trolls telling him he never said where they went. But for the themes of the show, it makes so much more sense for her to be making up the story, Kevin believes her because he wants to believe in something/someone, and it doesn't matter that it's not true if it helps her cope. If it's true, there really isn't much ambiguity, it just straight out answers all the questions and seems too tidy. The fact that we get flashes of Nora's kids and other things and when she's telling the story we see flashes to her getting into the machine, but we never see a glimpse of her going through seems to make sense that it didn't really happen. Of course this would be a cinematic thing to shoot, her walking around a vacant Mapleton, tying the beginning and end of the series together, but they don't do it because it's a lie.
Also interesting to note in interviews Lindelof keeps saying that if you know something isn't true but someone else tells the story and they believe it, does that make it true? Which also seems to support the notion that Lindelof believes its a lie but Mimi and Carrie believe it's true (none of those 3 ever discussed if it was true or a lie, each made up their own version that worked for them), so does that make it more true? The answer is: there is no answer. But I believe Lindelof believes it's not true, whether that makes it more definitive than Carrie or Mimi's version is up for debate.
Even the last shot of all the birds flying back to the roof is a great wink to the audience because WE keep wanting to believe in something greater, higher, more romantic so when it seems like these birds are really going to deliver these love notes into the world instead of coming back to Nora's, we want to believe that. But when they just return home, it's like, but of course they do. But for that moment they didn't, you believed. That's in our nature.
I will miss this show. And thrilled to see Lindelof getting his reputation back and can't wait to see what he does next.