I have no idea what this really means, but it's interesting to speculate on such a provocative statement. I adore Sebok, but I think he was more of a mystic and visionary than someone who dispensed concrete technical advice to students. Many of the pianists who really benefitted from him already had concert careers.
The word 'quantity' in relation to sound I cannot help but parse scientifically. It could be referring to any of the measurements of wavelength, amplitude and waveform that characterize the air vibrations we hear as music, each part of which can be defined in terms of numerical quantities. Read this way, quantity does absolutely equal quality. I don't think that's what Sebok really meant, though - he was more of a psychologist than a physicist.
This does however relate to voicing. Voicing is of supreme importance. When it is said of a pianist that they have exquisite 'colors' or an immediately distinctive 'touch', I believe that this largely refers to their skill at voicing - in other words, at balancing the dynamics of simultaneous notes, over time. If this is done in a personal, logical way, with incredible precision and as a kind of organic outcome of how the present instant in the music relates to what comes before and after, then magical things start to happen. And music critics start writing in terms of 'flavors' and 'textures'. This is also why a good piano teacher can construct an entire lesson around two successive chords.