I’ve been on the slim side most of my life, but I definitely can and do gain weight when I don’t pay attention.
The one time I gained the most was as a Peace Corps volunteer, in a part of rural Africa with nearly no processed foods. I baked my own bread, made my own cheese, killed my own chickens and mostly ate fresh vegetables from the daily market. I got really into cooking, and spent much of the day perfecting elaborate multi-course meals. Nothing really had added sugar, desserts were rare (I don’t have a big sweet tooth, so I don’t bake), and the sugar I had in the house would last forever (I mostly used it to make the bread rise).
Gained maybe 40 lbs, which I lost pretty much immediately after leaving. I’m pretty sure it’s because I cooked (and ate) all day, and it was basically too hot to move.
Bumping this to highlight hidden sugar again:
The cafes serving drinks with 25 teaspoons of sugar per cup - Starbucks, etc
Fwiw, with the knowledge about how you can gain weight even within your daily calorie limit by maxing out on added sugar, for the first time I feel in control of my weight and it’s coming down gradually. This was a great experience for me and I hope others picked up some ideas 
There is a sense in which you are both right. The point is that too much sugar can change the number of calories you burn. Too much sugar leads to too much insulin, which is very efficient at turning carbs into fat and then your metabolic rate can go down and you burn fewer calories.
But it is not just sugar. Some carbs are turned into sugar very quickly and can have the same effect. This is what glycemic index is all about.
One day, they will know a lot more about nutrition than they do now. When I think of all the changes in nutritional advice I’ve seen over the years, I am amazed.
Well, my best understand now is once you hit your daily limit of sugar, the liver has no choice but to turn the rest into fat regardless of over all calorie intake.
So it’s wrong to think you have one daily meter limit (of calories), we do have two meters (the second is a sugar limit) and creating fat can be triggered independently of the other.