No, you're missing the point. His son is going into a store and buying the game with no adult supervision in sight. The cashier is selling M-rated game to a minor without asking for any kind of age verification. The cashier has no idea if permission was given or where the money came from. That's the point Thompson is trying to prove; that retailers don't really care that much about carding kids for video games. In my experience (working as a cashier at Target many many years ago), such a thing was not far from the truth back in the day. Might be different now.
Real issue is, look at it from a statistics point of view. Sample size of 1? Kind of a worthless "sting" if you ask me. If he had hundreds of kids all over the country doing secret shopper type stuff, it'd be a bit more substantial.