No, Mel Kiper didn't have to say anything. It was years upon years of easily watchable football games that showed the kid was the best in the nation, and had heart, leadership, and intelligence. And zero question he wanted to play the game. I didn't only become aware of Luck in the offseason after he left Stanford. Not only that, but his size, arm strength, relative mobility... yes, all of it was world-class.
Height and sample size matter. And you're ignoring some of the other things about Luck that Murray hasn't exhibited. Comfortable leadership in the face of adversity and less talented teams, 100% commitment to the game, football intelligence, etc.
I've mentioned his on the field play plenty of times. His receivers were absolute studs who were 5 yards off the defender on every throw. His offensive line was chock full of NFL talent, so he rarely got touched, and mostly had a clean pocket. I question his ability to throw into tight windows. Not to mention football character concerns.
Heisman.com lists Lamar as having the second best season for a Heisman winner in terms of yards from scrimmage, only to Koy Detmer, and if I recall correctly from looking last night, scored more overall touchdowns. To me, that's statistically better.