I heard him play this aria as an encore at the Schauspielhaus Berlin in 1993. It was so beautiful! One could feel that Gulda just loved every note of this music.
Pretty sure he's just playing all 3 staves of the vocal score... playing both hands (mostly left hand) of the piano part and then incorporating the top vocal stave melody
Gulda war schon toll, aber hat sich leider dann dem Zeitgeist angepasst... Das haben jüdische Pianisten nicht getan. Das muss man ihnen zu Gute halten. Auch wenn sie kopieren, das aber perfekt, ohne sich zu verbiegen!
to me yours is an accurate assessment , and very original. I for one appreciate the respect the jewish musicians usually display towards classical music.