That highly selective colleges have more (potentially) successful pre-meds is no surprise due to selection effects (i.e. they have selected stronger students in the first place).
But that does not necessarily mean that they have favorable treatment effects for pre-meds compared to less selective colleges. I.e. if a student who was admitted to Swarthmore and Wayne State (assuming similar net price to take that out of the equation), is it necessarily true that choosing one or the other will increase the chance of success getting into medical school (and, if so, which one)?