I guess the point is that you can survive that long without food, but you won’t necessarily be at full operating capacity.
Heck, if I get busy and don;t get lunch until after 2pm (I don’t usually eat breakfast), then I start getting a headache.
I can’t imagine how I’d be if I waited until even later to eat my first meal of the day.

yes you can go 3 weeks without food.  That does mean your body starts effectively eating its self to survive.  Those numbers do vary based on environment. In a desert you live less than three days without water. You live less than 3 weeks without food if you are doing strenuous activity. And so on.

Yep, been watching a lot of Dual Survival on Discovery channel.
There are some good tidbits on there if you want to run a survivalist campaign.