The only scenario in which that would make sense is the one in which the calories removed from one’s diet (by switching to zero-calorie) are not replaced by something else.

And without meticulous calorie tracking (which is something that basically no overweight person does), I’d just assume those calories are made up elsewhere.

And even if those calories are not replaced by something else, I suspect that the switch to zero-calorie at most just decreases the overall caloric surplus one is still in. A can of Dr Pepper is 150 calories. So a person who’s consistently in a ~600 calorie surplus (very common) could switch out their 3 Dr Pepper a day habit for Diet DP and still be 150 over. That person will keep gaining weight even with a switch to diet.