Never mind Netflix: the iPhone 8 and 8 Plus don't have HDR displays, the iPhone X does
So, why did Netflix list the two new iPhones in its mobile HDR roster? Well, the same reason that the G6 popped up as the first phone that covered the Netflix Dolby Vision display requirements, despite that it doesn't have a panel that can, say, do a 1000 nits of brightness. Apple has even reached out to Mashable and clarified that iPhone 8 and 8 Plus adopters "will see visual enhancements to dynamic range, contrast, and wide color gamut when playing Dolby Vision or HDR 10 content from their respective content providers, but it will not be at the full level of HDR visual fidelity as it'll be on the iPhone X, which does have an HDR screen."
There you have it, you do get a boost in color gamut, contrast and other enhancements when you watch HDR Netflix shows with your fancy Premium subscription on the iPhone 8 or 8 Plus, and that could be all that matters to the untrained eye. The G6 panel also doesn't fully cover the Dolby Vision standard, but the folks from Dolby are on record saying about their mobile certification that "our colorists actually tune [the LCD screen] so it maps with the displays used by content creators."
The OLED display panels have much higher contrast ratio, though, which is one of the HDR standard requirements, so that might be why Apple lists only the iPhone X as having an HDR-worthy display, as you can see below, while the other new iPhones only "support" it.