Do they even own the URL? Is no one else allowed to post the same URL (even disregarding how/by whom this would be enforced)?

Well I imagine opensea at least prevents url collisions on their own service, but yes as the article demonstrates someone could sell the same url on several services while changing what that url points to whenever they like. I think most of the time the url points to the marketplace itself though?

So I suppose it is more accurate to say they own that particular citation of the url embedded in the blockchain, for certain values of own.