I see what you're saying. But think about how many people had to see the ad in the different papers that he took ads out in, and he got 15 orders since the ads starting running. And there's no way to know if those orders were a result of the ads, especially since he was obviously already getting plenty of sales from some other means of marketing.
You are right about one thing, all those people will see your ad, but how likely are they to go to your website, because they saw your ad. And even if they go, how likely are they to buy. Those are the questions.
You would be marketing your shirts across many demographics, and many of which have absolutely no interest in your ad. Direct mailings are just that: DIRECT. You get to pick your demographic, people you know are interested in your type of products and send them much more than what they could ever see in a newspaper ad.
Also, looking at the nationwide advertising link you reference earlier, for the money, direct mail would get a better response than advertising in those publications.
If you want something more than a simple text ad, the rates start to get high very quickly! And simple text ads about your site with the url, just isn't going to get much response in a free circular.