I cannot actually recall Dr. MacKenzie writing ANYTHING about the 18th hole or finishing hole in any of his articles or books.
He never said so directly, but I believe he thought more about match play than medal play. If that's so, there is no such thing as a "finishing hole" in match play, and the 18th is often not even part of the match.
I think it is fair to say, from observation, that he didn't put extra emphasis on the 18th, or even have a definite idea what type of hole should be last. When the ground wasn't particularly compelling I think he tended toward a longer par-4 (Alwoodley, Moortown, Augusta, Royal Melbourne), but obviously Cypress Point and Pasatiempo are examples that he wasn't bound to such a finish.
Just off the top of my head, I can't recall that he finished many courses with a par 5 -- Lahinch is the only one I can think of, and there he was just using the existing 18th hole.