Yes. The author of the article is barging through an open door: It's very likely that those Game of Thrones episodes had individual hidden watermarking, and that using those they can tell exactly who leaked it.
The author jumped to the conclusion that those don't exist, apparently because they aren't visible (but that's the point!) and that there is a visible watermark which the author apparently assumed was the only one (but it almost certainly isn't).
The author jumped to the conclusion that those don't exist, apparently because they aren't visible (but that's the point!) and that there is a visible watermark which the author apparently assumed was the only one (but it almost certainly isn't).