The reason I say felt first is that usually not all parts of the sweater felt up the same. If a different yarn or weave was used some parts will be tighter/looser than others. For example on the pink purse, the bottom part of the purse where it's all wavy was how the bottom of the sweater felted up, the body of the sweater was not wavy but tight, I just chose to integrate it into the design. If I had made the purse first it would have felted up weird.
Same problem with the Eddie Bauer sweater, the arm bands and waist bands didn't felt up the same, so if I had cut and sewed first it would have been a mess. Trims, different yarns, button holes, etc can all make the area they are in felt up differently. I felt it worked out better to felt first and then sew.
