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.)
Like yeah, they maybe can't, are you disabled? No? Well then.
It is nevertheless eminently possible to walk and bike more.