You are correct OP in that it is to limit the amount of carbs etc you consume. The basic premise of SW is that, if we eat to satisfy our appetites, eat when hungry, stop when comfortably full (not stuffed) then the volume/weight of food that we eat is remarkably consistent over time.
So if you eat a decent amount of superfree foods, which is most veg and fruit, this will limit the amount of calories that you eat, because they are lower in calories than other free foods like meat, fish, pasta and eggs, so will help you lose weight.
Some people might have meals that are made up of free foods, but don’t contain any superfree vegetables, eg plain omelette, steak and SW chips, beans on toast. But as this isn’t in the spirit of the plan, whether they would actually lose any weight would probably depend on their weight and activity levels. My consultant always encourages plenty of speed foods, to speed up weight loss.
I just treat the 'third superspeed foods' as a guide. There is no need to have a third of your plate visibly superfree, ie made up of separate salad or vegetables, if you already have a good amount of vegetables cooked into the meal. So if you include plenty of vegetables in your curries, stews, Bolognese sauces etc – your superfree is already included.
If I make Bolognese sauce for example, as well as lean mince, I will put mushrooms, carrots, celery and onions in there as well as tomato puree and passata. If you look at the pile of prepared veg, it is bigger than the meat, so probably more than a third of the meal, if you don‘t have loads of pasta.