Your first sentence is interesting, why do you think that?

I got a 2:1 from Imperial, guess what I think is on par with top 5% at Harvard ;)

I'm only kidding, but more seriously it's certainly not all equal, and few are really in a position to compare any two, nevermind several - it's a bit of a joke that we then make 'a 2:1 in an engineering or related subject' or 'at least 3.5 GPA' or whatever a requirement as though the institution and cohort doesn't make a difference at all.

Harvard takes in a lot of students on the basis of athletics, legacy, and other non-academic considerations. While I've seen students admitted to specific colleges in Oxford due to family ties, that has always been a matter of which college a student attends; I've never seen someone get in that way who wasn't already going to be admitted to the University on academic grounds.

If you look at the top 1/3 of Harvard students who deserve to be admitted on academic grounds, you end up with a much smaller pool than "top 1/3 of Harvard students".

It’s not like the legacies that get in are unqualified. You can look at any of their resumes, they still have stellar grades, standardized test scores, extracurriculars and more. Legacies aren’t just dumb rich people, their parents just went to that top tier school (and I’m sure you can imagine just having parents that highly educated can be a leg up in how they raise you their entire life). In fact, legacies can often be overqualified and should go to a more competitive school but are guided into the legacy school by their parents / guidance counselors.

At Harvard, about 12-15% of recent classes are legacy. Zero are admitted on the basis of athletics alone, that is literally the tenet on which the Ivy League was founded.

Even if you were right, though, it’d be an odd take — even if 25% (or 50%!) of Harvard students were admitted noncompetitively, they would simply not be part of the top 5% of Harvard students? Like they’re not diluting the top 5%, they’re just not in it.

I stand by “top 5% of Harvard” is a higher bar than “top 1/3 of Oxford”. But I will grant you that these are all based on GPA anyway, a rather noisy measure of even academic ability, let alone anything else we might care about.