Another thing to note is the survey indicates "Selectivity of UG institution"
not rigor of UG institution. There's a difference and people often lump the two together without realizing it.
There also many different meanings of this. Alot of private schools might not "give significantly more leeway" per se to people from top schools for a lower UG GPA, but rather certain top schools are "feeders" to certain private schools. So in a complete hypothetical made up example a school like Yale might not really look at UGs from Duke and ECU that much differently, but if they see a Yale or Harvard grad applying, suddenly they look at them very differently from other schools and are willing to overlook more of their flaws. Yet on that AAMC survey, that would still mean they account for undergrad significantly, just not in the way many might think
Likewise, I dont buy that say a SUNY looks at all UG institutions the same which is what some might imply by looking at that chart. Schools like SUNYs get a ton of applications from people in New York who go to top private schools, particularly those in New York like Cornell and Columbia. They have a good idea of what they are getting from those top UG schools. If they know that those with lower GPAs from Cornell tend to do just fine in their school, there's a good chance they'll be more willing to overlook lower GPAs than they would from generic State U even though technically they place "minimal importance in UG selectivity".
Interestingly, Miami releases the way they "give points" to applicants. UG prestige is given 30 points out of 300. MCAT and GPA 45 each. LORs and Essays 45. Patient contact experience 60. They also list where the people from their class came from; interestingly there isnt much representation from top schools(although almost every top school individually is represented) suggesting those "30 points" awarded for UG dont amount to that much. Note Miami really doesnt get much if any funding from the state anymore so they largely function as a "private school" for purposes of that AAMC survey.
http://admissions.med.miami.edu/md-programs/general-md/class-profile