"What’s wrong with saying that paleo is about lean meat?"
Because it's not! "Lean meat" makes people think it's turkey, skinless chicken breasts and tuna all the time. It's much more about red meat - grass fed, sure - but ruminants, definitely (and some fish, I guess).
Dr. Mike Eades take on Paleolithic man's preference for fatty meat:
"During Paleolithic times, man primarily subsisted by hunting. The preferred food was large game animals, and Paleolithic man, a skilled hunter, wiped most of them out. And not just the large grazing animals. Paleolithic man completely decimated the Cave bear. As you can see from the photo of my Cave bear skull below (from a slide I use in presentations), these were enormous animals that didn’t go down easily. Cave bear, like all bears, had high levels of body fat, which must have been highly desired because these ferocious animals were hunted to extinction about 15,000 years ago by people wielding little more than pointed sticks."