She's disconnected from her musical origins in Peru. After the Bernstein scene when she cries and leaves the viewer thinks "oh she's returned to Peru and to her musical origins". But now, she's come to talk the same bullshit she had before, but this time she's conducting some anime shit. The flashback would take us back somewhere she won't return to. Also, this flashback seems like an odd decision that doesn't add to the movie. I think we understand that Lydia Tar is filled with jealousy and competition and that whatever happened, she definitely abused her power for her own convenience. Isn't that enough? Why would we need a flashback in addition to all of that?