Ah I see what you mean. I think the author of that article is making the point that Merge is about creating trees, not appending words together in a more traditional sense. But S-expressions can be used to express trees, so when you're operating on trees notated as S-expressions, adding a parent node to two expressions looks like you're concatenating them.

The author is just trying to say that Merge is doing the same thing we're talking about cons doing to S-expressions to a tree and noting that it creates hierarchy. Eg a new Merge says I've created a new top-level node that is a parent for the two inputs. A second Merge says I've made the first input c-command both inputs of my first Merge and created a new top-level node.