Agree. Writer of the article got it wrong. You need it on your person, not in the car.
I think the medical POA is more useful for enabling parents to talk to the doctors/nurses. As in the example in the article, if your 18+ kid is in emergency surgery and you are 400 miles away and call the hospital for info, they’ll tell you to get lost. If you have the medical POA or HIPAA release, you can fax it or email it over to them along with a photo of your driver’s license.