All of the flashbacks that the audience sees are OA's first-hand retelling. She witnessed them or she would have reasonably been told by her fellow captives (i.e. Homer's NDEs and trip to Cuba). And it's important that the audience accepts that the flashbacks are her words, or the open-ended finale isn't open-ended at all. We are supposed to perceive that OA is an unreliable narrator so that we don't know whether or not her story is true.
However, the scene between Hap and his colleague who is also studying NDEs doesn't fit into the first-hand narrative. OA did not witness it, and Hap would have had no reason to relay that story to her. The audience certainly doesn't witness Hap telling anyone about that incident. So why do we, the audience, get to see it? Because it doesn't fit in with the unreliable narrator story structure, we can only assume that it happened, and thus everything that OA relayed also had to have happened. It totally destroys the premise of an unreliable narrator.