its all speculation at the moment. However going off all the off axis colour shift complaints, lower PPI and lower brightness i'm going to bet its not the same generation as the S8. It seems likely its an S6 panel with a modified sub pixel arrangement and Apples colour calibration.
The answer is actually neither. It is a display that was produced by Samsung, to be exactly what Apple asked for. The resolution was chosen by Apple and is very in line with what their resolution bumps have been in the past (they want to leave room for another bump in the future). The color profile fits what Apple has been doing the last few years, which is bumping up their saturation/contrast, in this case 130% of SRGB. Samsung does take a different approach with their own, up to 200% sRGB as explained earlier in the thread, however for some, this may actually push beyond the
appealing look of "full" colors, and therefore may seem over-saturated, based on opinion. Samsung provides options for these preferences, and lots of reviews fail to compare various modes, therefore subsequent feedback tends to bring on even more fractured opinions due to the partial information.
And by the way, at this point display superiority is now becoming preference towards color profiles. Expect this trend to continue accelerating over the next couple years.
Btw I call BS on the assertion that typical usage brightness is higher on the X's display, as they mention nothing about specifically turning off "Auto-Brightness" on the iPhone, which is found under general settings-accessibility-display accommodations, and if you don't turn off this hidden setting, adjusting the manual slider to the max will indeed benefit from the "boosted" auto-brightness feature.
Maybe I'm reaching, but no one will really know
Either way, the X has a great display, for sure.