Well presumably if they are alumni then they were given a high enough rating to get in so the numbers are skewed. Unless this is alumni from other universities.

No, Harvard alumni interviewing candidates is part of the admissions process. The alumni rated Asian candidates as being just as personable as candidates from other races. Since the alumni are volunteers from all walks of life (other than the obvious fact that they all went to Harvard), the idea is that they aren't affected by whatever is going on inside the admissions office, and the disparity between alumni ratings and admissions office ratings is evidence of systemic bias against Asians.