That doesn't mean the t-shirt won't last more than 6 months, just that the shirt has been worn 6 months with no issues. If the shirt was 2 years old, he may tell you the same thing.
Again, thin shirts don't equal "poor quality". It's old-world thinking, like people who still say polyester isn't good because it's easier for them to catch fire, or "bigger diamonds are better". I don't rate food quality the same way, either.
A thin t-shirt may be very poor quality, but can also be wonderful quality. In fact, I'd argue that "strength" doesn't equate to "quality" either. Even if a thick t-shirt offers better resistance to damage from pulling or tugging, is that how you rate "quality"? Quality of what, strength? I'm not saying you're wrong. A cheap, thick souvenir t-shirt would hold up better to physical tugs and pulling because, like you and ErikCLDR said, they're thicker, and there's a larger number of threads used to make the shirt. However, for general wear, does it matter?
I own a few t-shirts and jumpers from an American company called American Apparel. It's obvious that they're made from a high quality material, and the thinnest shirts hold up as well as any "thick" t-shirts from other brands. By "hold up", I mean they'll withstand the laundry machine and general usage, not a Tug-of-War. They don't need to withstand it, and I wouldn't notice or care if they did. I'd take my American Apparel polyester-cotton blend t-shirts over anything from AF or AE, even if they're plain and boring as hell.
