Having an extra negative 400 to 500 calorie offset on a daily basis can be incredibly helpful considering a lot of creeping weight gain is caused by a caloric surplus of only a couple hundred per day.
It doesn't work like he says. You can't just sum up the number of calories and convert it to pounds of fat. If you eat a few hundred calories too many you will rise in weight. But increasing your weight means your metabolism will increase. In the end you will reach an equilibrium were your weight is high enough that your metabolism equals your intake, and you will stop gaining weight.
Let's take a 5"10 male aged 25 weight 170 pounds. According to some calculator they will have a basal metabolism (how much you use if you stay in bed all day) of 1850 kcal / day. Lets say they eat 450 kcal / day too much. i.e. 2300 kcal / day. By running the metabolism calculator backwards we find that they stabilize on a weight of 245 pounds.
Thats a seriously overweight, to be sure, but it is also well below 200 pounds of extra weight.
Yes, there are 2nd order effects, and there are also a bunch of assumptions in basal rate calculations, and conversion efficiency etc. Your assumption that the current day surplus doesn't carry forward (i.e. that as someone gets heavier they done increase intake to account for increased basal metabolism) is also a pretty strong one.
Anyway I could have been clearer, but my point wasn't to suggest that you would end up exactly 200lb heavier, but that your calorie surplus adds up to roughly 200lb - exactly what happens to it depends on a lot of things.
This is response to someone who suggested they hadd seen a lot of clearly obese people who couldn't have got there on a few hundred extra kcal per day - however that doesn't seem accurate.