I think she's telling the truth yet the episode questions the concept of "truth" from start to finish. The reason I think she's telling the truth is that its the same situation from Season 2 reversed: Kevin says to her he's been talking to Patti Levin and Nora rejects his claims and leaves. She doesn't accept his truth. In the finale its Nora who needs Kevin to accept her truth - and he does.
And what's great is you can still debate Kevin's visions/delusions(?) - but the point is that Nora crusades to destroy false narratives, even if they give comfort. So it's possible to accept her monologue as truth and also to see it through the prism of accepting "belief." Maybe Nora really traveled somewhere - maybe she had a near death experience and
thinks she went somewhere - maybe she is lying (which she does, despite her protestations) - maybe she, as people do, has told herself the lie for so long she believes it.
It only matters in as much as you (each viewer) need to know, want to interpret, or feel happier with a particular narrative.
I think a big part of believing her comes down to Carrie Coon's absolute ****ing home-run performance. I heard that on set, after that take, Lindelof turned to the rest of the video village and said, "Well, I believe her."
He has given some great interviews in print and on podcasts shedding light on the process of this season while never confirming any theory over the other. Interestingly, he said that while the writer's room was unanimous about the ending, he also said that if you polled them all individually you would likely get different answers as to whether or not Nora is telling the truth. His only answer was "I want to believe her." Which I agree with.