It's true that HBO doesn't offer 4K HDR, but it has nothing to do with the episode. It comes down to your device(s), and service. I even compared when I was visiting my parents this past weekend. They own a B8 OLED, and are subbed to HBO through Direct TV (dish, so cable box). But they also own an Apple TV 4K, so I loaded up HBO NOW and used my login. Episode 3 was a dark, unwatchable mess on their cable box. Like, absolutely terrible. Arya's library scene is a good example. All the library shadows and details were crushed beyond recognition, you could barely see even Arya herself and the Walkers. I then switched over to the Apple TV 4K, and rewatched the library scene. Lots of detail, and shadows aren't crushed. The difference is night and day.