I am not going to walk 15 miles to Costco and carry a week of family groceries on my back. I use literally zero public transit. This advice is insane for people who live in a suburb.
Surely somewhere in that more-than-half-of-all-car-trips there are a few examples of car errands that could be easily replaced by bike, scooter or foot.
EDIT: incidentally even in the most car-centric dystopian suburb there are ways to incorporate more walking into your routine. The next time you go to a strip-mall or big box store you could park on the far side of the lot and walk to the door. A minor inconvenience in both time and effort, but you could probably add 1/2 mile or more of walking to your Saturday afternoon errands that way.
But I stand by the statement that most able-bodied Americans could find some way to incorporate more human-powered transport into their routine if they really wanted to. And in many cases they could do it without any adding any superfluous travel. I.e., it may be slower or even less convenient in some way, but it was a trip you were going to make anyway. You're just changing the mode of transit.
(Incidentally if it's truly too dangerous or inconvenient to walk across the Costco parking lot with a loaded shopping cart I think we're well on our way to the people from Wall-E.)
Like yeah, they maybe can't, are you disabled? No? Well then.
It is nevertheless eminently possible to walk and bike more.