I think it's fiber. Fiber has a moderating effect on uptake of food. Without it you get a rise in blood sugar, insulin response, and then glycogen is stored as fat.
However, it doesn't address for example why e.g. ground meat is less filling than whole meat, which is why I used it as an example. There appears to be something about the process of chewing that induces fullness. It could be something as simple as the additional time it takes to consume the food allows for our satiety to catch up with the amount of food we have eating. The article talks about "texture" instead, but it appears to refer to the same idea.
Fiber-rich foods alone don't seem sufficient to provide satiety either. Consume a head of broccoli and you will feel physically full, as your stomach will be stretched, but you will still want more food. This makes sense, as you have not consumed an adequate amount of calories or protein.
Potatoes are more interesting to consider than a head of broccoli though since they are one of the most satiating foods and it’s easier to ask why one food is satiating than why another is not.
You got that backwards, as others have pointed out. 80% chuck is 80% lean and 20% fat. Simply look at the different grades of ground meat and compare their appearance. 95% "extra lean" ground meat is a darker shade of brown while 70% "medium" ground meat is much whiter due to the higher fat content.
One thought I've had is that the key beneficial thing of healthy natural food is that it has intact cells.
Fresh apples need to be chewed much more than either juice or dried apples.