I agree that it does seem different. But the term "single-site" somewhat buries the fact that TikTok, YouTube or Instagram are not single source or single publisher. To a lesser degree even Amazon Marketplace could be viewed through that lens.

Each of these "apps" could be seen as alternatives to the web itself rather than being specific sites on the web.

In the context of this article though, I think the comparison is simply between search boxes that compete against each other for advertising money.