A bit more research yielded this too. From YLS Bulletin, which is official. It confirms that roughly half of Yale’s enrollment comes from Ivies or Stanford. 291 out of roughly 600 person enrollment. I don’t see how based on this information that you can conclude that what undergrad institution you attended is irrelevant to law school admissions, unless you posit that these elite schools inflate their grades more than other schools.
http://www.yale.edu/printer/bulletin/htmlfiles/law/law-school-students.html