So the article first states that lowering the mass will speed up the swing speed, then later makes the crux of an argument based on something that he already knows isn't the case. Peavey, I think it's talking about 2 different things. There's the bat speed, which is faster with cork, and then the batted-ball (i.e. speed of the ball after hit by bat), which is less with cork because of the lighter bat.