This came from the experience of many drivers who jump started their cars and then let them run to have the alternator charge up the battery. Modern alternators are not very good at this and many have a warning on the box that says: “This alternator is not a battery charger”.
The best way to charge up the battery is to use a charger in the first place, or, after running your errand, shut thecar off and hook up a regular charger. I carry a spare battery booster in the winter, just in case.