I don’t think he cared if his grades suffered.
You asked if a student who gets perfect test scores but not perfect homework scores is a genius. Maybe, maybe not. My son’s friend is lazy but smart. He aced his tests which helped compensate for his lower homework grades. He didn’t care if he got straight A’s. Not everyone cares about grades. Surely you are aware that not all geniuses want to go to college? A genius might be working at Burger King, or living in an Amazonian tribe. A genius is born, I would argue. (No doubt someone will argue back, lol.)
I agree with @Catcherinthetoast . Work ethic and intelligence are not the same thing. A great work ethic can certainly enhance intelligence though.