The fat body is not simply stored fat. In insects it functions rather like a combined liver and reserve tissue, running immune responses, detoxifying pesticides, synthesising vitellogenin and storing the reserves that carry a bee through winter. Feeding on it damages exactly the systems a colony needs to survive stress.
The 2019 evidence came from several lines: the location of feeding wounds on the bee's underside, the composition of mite waste, and the fact that mites survived and reproduced better on fat body tissue than on haemolymph in feeding trials. The finding reframed why varroa infestation is so damaging out of proportion to the volume consumed, and why infested bees show reduced pesticide tolerance and poor overwintering.
Much of varroa's harm also comes indirectly. The mite vectors viruses, notably deformed wing virus, and feeding wounds provide entry. Whether haemolymph makes up any meaningful part of the diet, and how much of colony collapse is attributable to feeding damage versus virus transmission, are still being argued.