You missed the point. How would this hypothetical Oswald on the sixth floor know how they got to the elevator, even if he saw them outside, heard the elevator, and recognized their voices below him? Pretty silly to make up a sighting at the domino room without knowing they went by the domino room.
Your false premise that he
”had to know” how they got to the elevator in order to say anything about seeing them is not the point. The other possibilities include:
1. He was just simply lying without really having any idea of where Norman and Jarman were at the time. But thought it would sound good as part of his lie.
2. He heard the elevator being used and stopping before it got to the sixth floor and seconds later heard Norman and Jarman below him. He deduced that in order to get onto the elevators Norman and Jarman would [most likely, if you insist] have been within sight of the door of the domino room. And he included this as part of his lie.
My point is that it wasn’t necessary for him to see Norman and Jarman outside in order to deduce that they would [most likely, if you insist] have been in view from the door of the domino room.