It's basically impossible to judge 1080p HDR video quality from an SDR 720p YouTube video. It's also extremely difficult to test black levels in a room that bright.
BTW, I tested the Wonder Woman trailer on the iPhone X and iPhone 7 Plus side by side in the store. They both looked great. Even the blacks looked similar, but because again, I was testing in the Apple Store under bright overhead lights, which is just about the worst place to test the best features of OLED.
IOW, trying to test contrast and black levels in such a bright environment is akin to trying to assess sound quality in that environment with 1500 other attendees talking to each other in the background.