I thought he was talking about attitude/expectations and how much of a difference those can have on the shape/course of someone's life.
l honestly agree: what is expected of you, what your family values, and eventually what you see yourself as capable of is probably the biggest difference between low-income groups and the middle class. It's easy to stop working once you reach the state you expect to live in. For my dad, that was "surviving with enough to buy cigarettes." For my mom, it was a terminal degree. They both got exactly what they aimed for. SES differences go beyond paychecks; there is a HUGE cultural component that too often gets overlooked. It's why I didn't mark myself as SES disadvantaged, even though we were quite destitute for much of my childhood. Despite that, I grew up in a household where education was highly valued, with all the benefits that come with that (homework help, plenty of reading, school districts factored into apartment searches) and I was expected not only to go to college, but to go to a great one and then any graduate program I needed for whatever career I decided upon. I would be the same person if I had gone with my father in the divorce, but I probably wouldn't be applying to med school, and not due to funds.
Edit: ah, saw the ending...I don't agree that it is med schools' fault, of course, but if they want more SES diversity, as they claim to, there are loads of things they could do to actually move closer to that goal than they are now. Like making the disadvantaged checkbox less of a possible landmine, adjusting the fee structure for apps, travel assistance for FAP applicants, etc.