“Is it within the realm of possibility that this guy could regularly earn this kind of money from actual gambling?”
No chance. The casinos would not let him in the door.
Years ago my grandmother used to brag about coming up to Tahoe and winning $300 over the weekend. However, if you talked to my aunts, she came up with $500.
Casinos don’t care how much you win if your losses are big also. Especially slots. A good gambler on slots can win more than lose most of the time but cannot win all of the time.
Big wins usually require proportionally big losses.