Good point, and I'm sure it's true. However, the group who learnt to improve their self-image were on an intervention plan which also covered exercise and emotional eating, amongst many other things. You cannot extrapolate from this trial that improving your self-image helps dieting. You can only conclude that this sort of intervention plan helps dieting. In order to come to the former conclusion, improving self-image would need to be the only thing that was different in the trial.
It's something I've been wondering about. We can all see that self-hatred is destructive, including when dieting, and that a shame mentality hampers successful dieting. One of the things I wonder about is how you're meant to love your body all the way through, which means loving it when it's fat and presumably refraining from feeling that the fatter version of you was bad when you're losing weight. Bit of a mental trick there. Maybe it's about focusing on the good parts and not thinking about "I look good now, so before I looked bad"?