If all depends on if they want to stop you or not. If they give you open shots, you might could score. If they REALLY want to stop you, you would never score.
The reason you can score on former nba players in the gym is because they absolutely don't care. If they were giving it their all they would completely shut you down.
You don't guard one person in a game... you have played I hope?
Screens, picks, loose balls, rebounds... you never have your defender "giving their all" to stop you from scoring. It isn't one on one. They help, they switch, they end up in the wrong place in transition... 100 ways to find yourself free from defense enough to score. You aren't necessarily scoring on your defender, you are hitting a shot that you found or earned.
One on one, sure - the scenario isn't one on one.