Correlation does not equal causation. Some of the strongest arms in the league have average or smaller-than-average hands. This proves that large hands aren't necessary for top flight arm strength. So controlling for arm strength, what use is the correlation? Arm talent can be evaluated without accounting for hand size.
Not sure why this is so difficult. Having big hands is better than having small hands for a position that requires you to hold onto the ball on every play. No one is saying it's a determining factor of anything. It's just another thing that scouts examine.

