I'm guessing that what they meant by "proper harmony" is just intonation: where thirds and fifths are expressed by small, integer ratios of frequencies (e.g., a fifth is 3:2 and a major third is 5:4).

A just intoned major third is about 14 cents flatter than a major third played on a 12 tone equal temperament tuned instrument (e.g., piano).

I'm not sure how much this matters in terms of having or not having perfect pitch though. Some people with perfect pitch can hear the difference between JI and 12TET and correctly their singing accordingly.