What are you talking about? His reasoning is good there, but I'm assuming you're saying it's bad? In fact it not only applies to the opposing defense but also to the offense too.
Example 1: a running back running for 2,000 yards with Wisconsin's OL may not be as good as a running back running for 1,800 yards with Indiana's offensive line. Because Wisconsin has a better offensive line.
Example 2: a running back running for 2,000 yards in the Big 12 may not be as good as a running back running for 1,800 yards in the Big Ten. Because the Big Ten has better run defense.
The difference between 2,000 and 1,800 is 200 but there are other factors involved besides just the difference in # of yards.
Now that said both players played in the Big Ten...