For reasons I don't recall, my dad bought a 1940 Chevy sedan when I was 14 in 1964. I think he paid less than 50 bucks for it. The car had rust problems but the interior was OK and it had cool running boards and rear suicide doors. I and my buddies used to to drive it around the 4 acres we lived on. I suppose my dad showed me how to shift it - once - and the rest was up to me. I don't recall having difficulty with it, but that was 62 years ago.
The study makes some sense to me. To drive a manual transmission you need to pay attention to a tach or your ears, you need a certain set of motor skills, and you always need to be aware of what gear you're in, the speed traffic is moving, and the next gear change you will need to make based on engine speed and vehicle speed. You have to be a lot more involved with the task of driving than you do with an automatic.
The only time I prefer an automatic to a stick is when I'm in a stop-and-go traffic jam. Otherwise I vastly prefer the stick. Any time I'm driving through town, an automatic transmission is almost never in the gear I want, especially when I need to go around a corner. I don't think most people ever think about using the gears in their automatic transmission car.
I do love the fact that we have a couple of cars with shift paddles (a BMW 328 and a Jeep Grand Cherokee with the 5.7L V8). We just finished a 5-day trip to Mesa Verde and the SW corner of Colorado around Durango and Silverton, and on 6 and 7 percent descents I used the BMW's shift paddles constantly. I see many people wearing out their brakes on downhills while I just coast along in an appropriate gear, using engine braking to reduce the amount of braking required.