This may indeed take months to shift, but it's not even close to the maximum possible damage to someone's finances from gambling.
Is this actually a thing? I've definitely massively overeaten for short periods before (e.g. Christmas dinner). But the effect on weight was very transient.
I've had this struggle for a few years, gaining about 30 lbs over the course of a few years.
What the GP said might not apply to everyone, but it does apply to those who have eaten to excess in the past and are now struggling to keep that excess weight off again.
I have also many times lost 2kg over a period of a few weeks by simply being lazy to calculate my calorie intake while excersising more than usual. Gaining weight takes ridiculously more effort for me than losing weight.
Which is of course trivial to do. That's one kebab extra per day.
You just don't eat caloricilly dense food at a high frequency and grossly underestimate how many calories someone can eat in a day [1]. Also, for most people losing 1kg a week is considered very fast weight loss at the border of what's recommended for safe caloric restriction.
That's a 10,000+ calorie surplus, you'd have to eat 1.5kg of straight mayonnaise and digest it all.
Sure, you can get the scale to shift by that much, I can do that in a few minutes by drinking 1.4L of water.
If you want to learn about actual realistic bodyweight tuning, look into the bodybuilding community, or any sport that has weight classes. Much more accurate information than using indirect measures of what people sort of felt like they sort of wanted to eat.