if they can pull DNA from ‘cow butter’ wouldn’t they also be able to potentially isolate human dna from scraping the skin of fingers on the rock as they painted with the butter?
That would be nice, but since the cow butter may not exist in large enough quantity, it's unlikely human DNA would remain in sufficient quantity (and quality). The cow butter wasn't ID'd at all, but is guessed to have been present because "a wide range of proteins in the pigment, including bovine albumin and casein" is present.