Forget the physics. A good chance there was psychology as the most important factor. Also this was the '70's and the quest for lightness was held in high regard. Lighter was potentially faster, therefore one wanted every edge possible.
By the way, the tires on the bike at the Merckx station are not original, they looked like #1 or #3 black tread Clements, The images of the hour record show him on white treaded tires, therefore #1's or #2's with white tread.
I always felt faster riding #1's, they were the best one could get, that and 10 atm pumped into them. And Urban legend was that the matt tread ones were better for Northbrook, so thats what I got.