In cooking, meat (and other things, like vegetables) can be said to 'sweat' when moisture exudes from the surface. It can be drawn to the surface by salt, can be squeezed out from the fibers due to their contraction from heat, or a combination of the two.
With meat, much moisture is driven in toward the center of the cut due to the surrounding heat of cooking. That's why resting is important. It gives the moisture time to migrate away from the center as the meat surfaces cool.
With vegs, one 'sweats' them in a pan, i.e., cooks in a little fat at lower temps, avoiding browning, in order to drive off the water vegs contain and concentrate their flavors, a basic technique that is something that would likely make Mill's recipe much better. As written, assuming one just throws everything together into a pot, the grated apple does nothing at all, and the aromatics (the onion and green pepper) don't add very much. The absence of fat means the fat soluble flavors in the sauce do not blend nor carry well.