The piano sensei probably used up most of their alloted time (which he said was 30 min.) to teach the girls how to play their parts of the song on the instruments that he brought, then in post they just edited those parts out, resulting in the "short" talk time. It would have been hella-ass boring if we would have had to have sat through all that (we all would have complained about it).
Anyways, I couldn't have but smile when Nakai was playing that pianica on the floor and the girls were saying how cute he was doing it.