I would guess the point of the other poster might be that even if students accepted to Harvard know that they can get an equivalent education elsewhere that they would still be motivated to go to Harvard over the other schools regardless. Unless there was a specific reason to go to the other schools (such as Cal Tech being a better science and engineering school than Harvard).

Perhaps an inference which could be drawn from this is that Harvard, as an institution itself, doesn't need to inflate grades in order to get and retain students. It could more harshly grade to no ill effect to itself. Whereas most other schools can't.

Harvard can also afford to annoy a significant number of the legacy donor class. Why they don't is not fully explained by comparison to other schools (who often actually need the money to stay open). A guess as to why they avoid annoying the donor class is camaraderie and opportunities (for themselves, and possibly also for students). The opportunities is a reason they would mostly share with other schools, the camaraderie, less so.

"If you think the explanation for why this happens at Harvard is different for the explanation for why this happens to most of the rest of the US, then you would want to explain what makes Harvard different. "