You didn't take into account the part right after that "Or at least the merchant would assume so."
So me and my wagon team have arrived at this place. As far as we can tell, there's just the depot and a hole in the ground. Not much to look at, anyway. I went into the hole to find the mayor so we can talk about a trade agreement - these guys are still living in the dirt layers! There's no statues, and what stone they do have hasn't been smoothed yet. Certainly no weapons aside from the woodcutter's axe. Their dining room is just a couple of tables and chairs tucked into a corner of one of the stockpiles. I don't hear any heavy industry aside from the miners - no smithies, no glassworks, no windmills or water wheels. There's a guy chipping away at some stone blocks to make a pretty crude-looking toy anvil; if that's the best they have for trade, I don't think we're going to be giving them much.
I mean really, should I expect to be frightened of these people?
In short, traders will have some knowledge of the details of your fort. If nothing else, they will have heard from other traders (and if they find a site that nobody's told them about, they can reasonably assume that it's brand-new). If we assume that the liaison talks with the traders, which is reasonable as they set prices for goods, then the traders will have detailed knowledge of everything the liaison has seen, which is pretty much every aspect of your fort. More generally, we can assume that the traders don't spend
all their time locked up in the depot; they wander around a bit and see the sights while they're in the area. That part's just not being simulated yet.
Even if the traders don't know specifics of your defenses, they'll know roughly how wealthy you are, which is a good indicator of how well-defended you are by whatever means (since wealth also means goblin depredations).