I don't think it's true for all activities. The author is a writer. I don't think you can grind story building or maybe you can but I doubt you will get anything actually good or creative out of it. Do you know of any painters that grind painting like you can grind exercise?
All this advice about discipline is good for stuff like keeping your house clean, going to the gym, sucking it up and doing your tasks that suck at big corp or any other task that sucks because it's boring or it physically hurts. It's boring because you already kind of know what needs to be done so there is no novelty to it, it doesn't need your creative input and if you are a creative person that sucks.
I care more about being satisfied with my life, getting pleasure out of it. That to me is being successful. I don't need a vacation home and a yacht or private jet so why would I grind and toil for extra money?