Martha Argerich may not have perfect pitch, but the idea that she might not have good relative pitch, the ability to tell the difference between notes, is beyond absurd. If you can sing a tune accurately you have relative pitch, even if you don't know what key the tune is in, or the names of the intervals involved. If you can distinguish that there is a wrong note in a chord you have relative pitch, even if you haven't used it to develop the ability to determine which note is wrong. Is anyone suggesting Martha Argerich can't sing a tune or hear a wrong note when it is played? Does anyone think she would play a concert on a piano that had not been accurately tuned and not notice? Get a grip! My nine year old son, who can play a few simple pieces can hear that the high B natural he plays at the end of Scarborough Fair is not "right" on our piano. That means he has decent relative pitch, even if he has not yet acquired the mechanical technique of identifying what's wrong with the pitch.
Relative pitch means hearing the differences between notes. Good relative pitch means hearing the intervals with some precision. If I sing "Here comes the Bride", and deliberately sing the second note a bit flat, most people who listen to music will be able to hear that it's flat, even if they don't know the interval is called in English a "fourth." Identifying intervals by name, or taking melodic or harmonic dictation, are simply skills that anyone who already has relative pitch can develop with practice.
WHAT are people thinking?
Sasha