I think going from almost no exercise to walking a few miles a day probably has enough of a positive effect as to help some people break the "feel like crap > e

I think going from almost no exercise to walking a few miles a day probably has enough of a positive effect as to help some people break the "feel like crap > eat comfort and junk food to feel better > feel like crap later" death spiral. If they had reached some sort of "stable" level of obesity, that is if they had plateaud their weight gain naturally, then a sudden consistent 400kcal deficit would result in some change. Assuming they didn't eat more to make up for it.