This is actually their DRM speaking. If you watch it on a Linux device or basically anything that isn’t a smart TV on the latest OS, they limit you to a 720p low bitrate stream, even if you pay for 4k. (See Louis Rossman’s video on the topic)
OP said they're using an Apple TV, which most definitely supports the 4K DRM.
OP said they're using an Apple TV, which most definitely supports the 4K DRM.