The shooters likely spend the vast majority of their time shooting a single pistol, so of course their performance would be better doing this... it is like saying shooting right handed is better, when the shooter who does the test is right handed! Not that I dispute the results personally, because I shoot with one pistol nearly all the time also, but there are trick shooters who are very good with two pistols. The correct comparison would be a sample of shooters who excel at each method and have sufficient training directly related to the task being measured (walking approach to target from 30 yards).
You’ll notice that the shooter in the ‘dual pistol’ case extends and retracts whatever arm is up on line. Of course this takes a lot of time. In the single shooter’s case I bet the extension was always the same. I’d like to see a test of both arms extended against a single shooter in that case.