Well hearing Greenberg mention that he thought melodies was being articifially constrain by "arbitrary" rules of tonality. Also his belief that the only people didn't like his 12 tone technique is that people aren't used to it and it will just take time for people to think it as pleasurable music (which by the way Greenberg who is a fan of Schoenberg admitted that he was wrong in that belief and that his work will forever sound modern. That his work will just be as difficult today as it was when it first invented). It's hard to conclude otherwise.
If he really believed in tonality as part of the language of music. he would have used atonality to expressed discord/chaos/despair and negative emotions and used the traditional tonal system to express other emotions.
He clearly didn't do that in his Piano concerto.
Essentially his Piano concerto seems to reveal someone with a belief that by varying phrase structure, rhythm, melodies, form, timbre he could express any emotions he wanted and completely ignore harmony/tonality in the role of the language of music.