Yes, and for the most part, that's exactly what he does. However, in order to trigger the vocal harmony processor, there must be a midi signal from the device, which is set for his left hand chords. The processor needs the chord information in order to provide the corrections for the harmony. This is also the case with an external vocal harmonizer - it requires the midi information from the keyboard's chords being played in order to function properly.
All the best,
Gary