“he told me to work on a specific muscle group per workout to make the workout more effective and i was wondering if he is correct?”
no its not . just split your days into upper and lower body days.
For upper body, id say work on your shoulders and chest
For lower body, work on quads, hamstrings, and glutes
muscles such as biceps, etc shouldnt be focused on since they will only add extra weight, while it is true that all muscles play a role in sprinting, some muscles dont play a significant role, and they should be worked when your doing stuff like bench press, military press etc.