To me, "grit" requires adversity instead of simply overall breadth. It's quite easy for us humans to do something if everything is going well.
What about when it's not going well? Do we have enough to persevere through all times, including hard ones? Your startup is failing? Your research is yielding no fruit? You can't get that promotion even though you've tried 3 times in 3 years?
Pushing through adversity and continuing the grind in the face of simply giving up is better defined as "having grit".
The problem simply is, maybe it is all just a waste? But that's not having grit!
Under the author's premise, you should be trying 1000% at all areas of your life all the time. Unrealistic.