“There’s overlap in the basics” is exactly what I meant. A person with a 4.0 from a rigorous high school and a 36 ACT statistically has a better chance of getting in to both Stanford and Harvard than a person with a 3.8 from a mediocre high school and a 33 ACT score. It is more common for someone who is accepted to Stanford to find out that they also got into Harvard than it is for someone rejected by Stanford to find out that they got into Harvard. So there is a statistical correlation - but it has nothing to do with any coordination between the admissions offices of Stanford and Harvard.
And it doesn’t settle the fate of any individual application, because none of us are statistics.