Complex sentence: While Seabiscuit had not lived up to his racing potential, he was not the poor performer that Fitzsimmons had taken him for or as depicted in the 2003 movie--many thoroughbred racehorses never break their maiden, and fail to win even one race.
Split into simpler sentences: While Seabiscuit had not lived up to his racing potential, he was not the poor performer that Fitzsimmons had taken him for. Many thoroughbred racehorses never break into racing their maiden year and fail to win even one race.