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.
A "buy in bulk" warehouse store is of course one of the least reasonable examples you could have selected, but per the Bureau of Transportation Statistics 52% of car trips in the US are less than 3 miles, and 28% are less than 1 mile. Those are both very walk-able and bike-able distances.
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.
A quarter mile of dodging cars with an ungainly, loaded shopping cart sure does sound like an "inconvenience," and not one I would recommend to a friend to improve their fitness.
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.)
You can't win debates with such people, about biking or walking; there's always another reason why they (and all other good and upstanding Americans) can't possibly walk or bike. When they run out of their own made up reasons, it's "well what about disabled people!"
Like yeah, they maybe can't, are you disabled? No? Well then.
Not sure if you or OP are in the US or not, but American cities have zoning laws and infrastructure that are designed entirely around the car. Aside from a few exceptions, towns in the US aren't like European towns in the slightest. It really is often the case that nothing is walkable or bikeable. The "Not Just Bikes" YouTube channel goes into all this if you are interested in that sort of stuff.
