Perhaps I've misunderstood, but 80% ground beef has 20% fat, making it less than a 25-30% far tenderloin (can't confirm that number, just going with yours), which seems to invalidate your claim.
Yeah, you're right. I'm not sure how I butchered that so badly. The thought of ground beef being 80-95% fat even sounded absurd to me as I was writing it.
Not that person, but I believe you're right. I think it's to do with muscle fiber - breaking the connections between the meat is fully pre-empted by grinding, and I theorize a connection between the mechanics of chewing and satiety.