The supposition is that a more expressive language lets you do more with a single line of code on average than a less expressive language. The second supposition is that commits tend to be done to gather code expressing a single chunk of functionality in a program, so that on the average commits have the same utility in terms of what they contribute to the source project.
It's clear from this, I would think, why therefore length-of-commit is supposed to be a good proxy for measuring expressiveness.
To be clear -- the reason that it is obvious that I and the author disagree with you on the first case is because your objection was a) an elementary one and a consideration important to all such investigations, therefore it would be considered by anyone doing such an investigation or analyzing one and b) we were disagreeing with you anyway.
It's clear from this, I would think, why therefore length-of-commit is supposed to be a good proxy for measuring expressiveness.
To be clear -- the reason that it is obvious that I and the author disagree with you on the first case is because your objection was a) an elementary one and a consideration important to all such investigations, therefore it would be considered by anyone doing such an investigation or analyzing one and b) we were disagreeing with you anyway.