The kind of people who can get into an Ivy League university would probably outperform the other group even without ever actually going to the university: statistically (stereotypically?), they come from a rich and stable background, have strong network of rich and influential friends and acquaintances already, have access to capital, they don't color outside the lines, and when they do, they know how to cover their a.. legally speaking, and of course, they are probably not dumb, either.

Yes, going to these universities probably makes some difference, but it probably doesn't explain all the difference, so if you take poor but talented kids and have them go to these universities, I expect the salary difference is much smaller.