Mozart is unaware that he has just humilliated Salieri; he honestly thought he was helping him.
In spite of taking, ahem, liberties with the biographies, it remains an enduring, always enjoyable film.
Mozart really was rather callow towards his fellow musicians. His letters are filled with accounts of other musicians coming up short in some way and him describing how he ‘laughed till I fell over’ or ‘could have died laughing’. He was not a good colleague.