You're pretty much correct. AF is a heart rhythm abnormality, and if it persists, like it seems to here, it renders the heart unable to pump blood. It appears he might have been undergoing a prostate exam when this happened, since that's the usual reason for a digital rectal exam. You'd have to assume the doctor knew they had this condition to begin with.

The heart was going “kethumpathumpadump,” the doctor put a finger up the guy’s butt, and then his heart reset to a normal “lub dub, lub dub.”

Not familiar with the paper this is from, but Atrial Fibrilation isn't a heart attack (it can cause one, or a stroke). The human heart has 4 chambers, the left and right atria are on top and the left and right ventricles are on the bottom. In super layman's terms, blood enters the heart from the lungs into the left atria and from the body into the right atria, passes through valves into the ventricles, and then is passed into the body (from the left ventricle) or the lungs (right ventricle). Normally the atria squeeze, there's a slight pause to allow blood to enter the ventricles, then the ventricles squeeze. In A-fib, the atria just quiver, they don't squeeze. It can be fairly benign and people can walk around for months without knowing they're in A-fib because the blood will just drop into the ventricles and the ventricles do the work of pumping blood out into the lungs and the body. But the problem is that in A-fib some blood tends to hang out in the atria and it doesn't completely empty, so eventually it can clot and now you have a huge clot hanging out inside your heart. If that clot decides to move it can go out into your body and end up in one of the coronary arteries (the arteries on the outside of your heart that supply your heart muscle itself with blood) and cause a heart attack, it can go to your brain and cause a stroke, or it can go into the lungs and cause a pulmonary embolism (PE). So usually people with A-fib are put on blood thinners to keep the clotting from occurring, or if the A-fib is too high of a rate (rapid A-fib) they're sometimes given medication or cardioverted (shocked) out of it.

Like another commenter stated, in guessing they stimulated the vagus nerve which converted his heart rhythm into sinus rhythm, which is the normal heart rhythm.

I've been told in the past that if you feel yourself going into a fib, you should push like your trying to shit to bring you out of it.