“However, I directly questioned a seasoned adcom at one of D’s schools for which the majority of students come from our state.”
So what college are we talking about? Most schools don’t need to do this and don’t do this. But the highly selective ones absolutely do this.
For example, it is very easy to determine what math classes a particular HS offers. So it is therefore easy to discriminate between a kid who only took regular math classes and got an A. Versus a kid that got an A in AP Calc BC.
Who then also got a 5 on the Calc BC AP test. Who then also got an 800 on the Math 2 SAT subject test. Who then also had a rec letter from the math teacher saying the kid is the best math student he has ever seen in 20 years of teaching.
Nobody gets into Stanford or MIT on the basis of whether their HS has grade inflation or not.