Grit, or willpower, or whatever you want to name it isn't a unique, constant value. There are plenty of athletes who could spent hours training every day but are overcome by addictions. People who grind at work but cannot fill paperwork to save their life. That will diligently do something for months then stops after an unexpected interruption.

There's probably generally a bit of correlation. But just because someone can be very focused and go to extreme lengths in one aspect of their life doesn't mean they can consistently do it in every aspect of their life.