the fact they are using the legacy backdoor to get in, instead of the great academics route... kinda would dictate they aren't on the same level of "smarts".
People only use the backdoor when the front one wont work.
I agree, but to clarify a point: the "nearly half" number includes athletes, legacy admits, children of faculty and big doners, whether or not they would have otherwise gotten in on academic merits. The authors estimate that only a quarter would have. I didn't read deeply enough to see if they broke that number by legacy admits only.
Does the study say that they "used the legacy backdoor"? I think most college applications just ask the applicant if any family members attended or teach at the institution. It would be pretty foolish to leave that blank if you had family members that had gone or taught there.
People only use the backdoor when the front one wont work.