This statement is a bit misleading, as the criteria get to be hair-splittingly narrow when you are talking about a school with a sub 5% acceptance rate -- you could fill the whole class with valedictorians. Nevertheless, my experience is that in terms of finding a 'better' candidate:

Legacy -- legacy preference is pitched as a tiebreaker. Most of the legacies are actually quite good, and many are exceptional, so perhaps 30-50% got in over some 'better' kid. But in many cases, those slots represent something like geographic diversity, or a legacy kid of a minority or a kid of some famous person, and they generally don't take dolts. So this is not a huge tip.

FacBrat -- The kids of professors tend to be extremely and sometimes extraordinarily good -- their parents are Harvard professors, and that tends to rub off. Staff members less so, but it is politically hard to reject them if you want to keep their parents. There are more in the second category than in the first. So perhaps 60% of the kids get in based on this tip.

Donor -- Not a lot of these that I know of. Even in the 1930s, the son of the President of IBM got rejected from Harvard and Princeton because he was a goof off. I seriously doubt that there are more than 10-20 kids/year who get in this way who wouldn't otherwise.

Athlete -- This is where very few of these kids would get in if they were in the general pool -- 10% max I'd guess. A lot of them are very good, but that level of dedication to sport tends to eat time that could have been used for academics or other worthwhile pursuits.

I've always been amazed that they recruit for Golf, Squash, Crew, Fencing, Diving, Tennis, Lax, and Water Polo... these sports are limited to prep schools and rich suburban districts ... not exactly equitable.