I think he means, and please correct me if I am wrong, but I think it goes something like this, I wasn't present when John Wilkes Booth murdered Abraham Lincoln yet in my minds eye I have a pretty good memory of what I think it must have looked like based on things I have read, TV and movies reenactments and so forth. I wasn't at the Pearl Harbor attack or JFK's assassination but I have read and seen enough video to give myself a working memory of the events.
At least that was the way I understood the earlier post.
That's pretty much what I'm talking about, but in more detail considering his closeness to the situations. He's no doubt read the mission reports and files so much that he has memories of the events happening even if he wasn't present at the missions.
I wasn't actually in the OR when my mom had her cancer removed, but having met the doctors and one of the nurses, I have a memory of them doing the surgery. Not with details of the procedure, of course.