Unfortunately, in this situation, they're not kidding themselves, but conning others. We call that sandbagging here.
Generally, the ones who are kidding themselves are the ones who only post good scores to reduce their handicaps. . .
Try not taking any practice swings. I don't. I tested this a few years back at a practice green. I found my accuracy and distance control was better without a practice swing. I find it just distracts me. If I do take any to loosen up, it is while others are setting up to putt.
I pick my line and start direction from behind the ball, walk up, line up, settle my feet, look down at my ball, do a small forward press and putt. All I focus on after looking at the ball is the start direction. Try it.