The point isn't that the particular strategy for health and fitness is right or wrong, it's that the growth mindset causes the person (Bob?) to over-focus on that aspect of their life to the detriment of more important but less obvious ones, like personal relationships, being kind to others, or enjoying fallen leaves in autumn.
Bob spends his 20s trying to improve his bodyfat percentage, max his bench, get a Porsche, and build his 401k.
He misses out on his niece's early childhood, loses his highschool friendships, and breaks up with his girlfriend for a job opportunity in a different city.
This is like the printer company focusing on revenue instead of brand loyalty or its reputation.