The answer is simple. Take the same gun in both 9mm and 40 S&W and shoot them side by side. 9 out of 10 people will shoot the 9mm more accuratly in rapid fire and also enjoy the lessened recoil.
As as I was saying previously, such blanket statements ignore variables such as the build of the shooter as well as the firearm.
Sure, for the lowest common denominator of civilian shooters who tend to gravitate towards smaller, lighter, easier to conceal pistols/bullets and doesn't spend very much time at the range, the statement is fairly accurate.
However, for Law Enforcement officers who you'd hope are in better overall physical condition, carry full-size pistols which are typically heavy duty service-grade firearms, and spend more time shooting than your average Joe Shmoe who buys a firearm then proceeds to throw it directly into his holster or nightstand drawer before even taking it out to the range, not so much.
Out of a full-size pistol, the difference in felt recoil better 9mm and .40 S&W defensive loads can be downright negligible, especially in heavier all-metal service pistols like the Beretta 92/96 or SIG P226.
So even if modern police departments are packed full of lady cops and smaller framed gentlemen, I doubt that the difference between the 155-180gr .40 S&W loads they were carrying and the 124-147gr 9mm Luger +P+ loads they're currently carrying have an appreciable difference in felt recoil.
I believe that the large amount of rounds fired has more to do with the changeover from 6 shot revolvers to 15+ round semi auto pistols. One of the arguments against the switch was that officers would be more inclined to spray and pray during gunfights. From watching the numerous body cam footage now available, I believe that to be true in many cases. In some of the high round count videos that I've seen, one well aimed round from a Colt SAA would have done the job.
It would have been interesting to compare DA revolver involved gunfight videos if the technology existed back then.
Perhaps, but regardless of the reason, adequate training should prevent them from haphazardly slinging lead, both to mitigate the risk of collateral damage as well as to conserve ammo so that officers don't have to pause to reload in gunfights as often.