Oh I never said it's 'real', as I just don't know that for a fact and as it's really fast I doubt it... it's a planned film, anything can happen, na film trick. In her defense she was a very good pianist; she started when she was 3.
We must admit some of the things 'actors' did in really old films are pretty amazing. One of the first films to boggle me was 'Seven Brides for Seven Brothers' for the dancing.
PS - people have played 2 pianos LIVE.