Keep in mind that sometimes implementation details of technology can hide very important differences, so you need to normalize if you want to compare them. In this case, "pixel" does not have a uniform meaning, because it conceals what actually goes into making each unit of light. The original "Retina" displays were LCDs, high quality high DPI LCDs of course, but still LCDs in terms of the pixels being made from even individual sets of red, green and blue. In contrast the OLED in the iPhone X (and now successors) are "PenTile" designs, diamond pixels with a larger green element and smaller red/blue ones wedged in (taking advantage of the human eye's general spectrum response to some extent). This asymmetric arrangement though can mean that an LCD and PenTile display of equivalent "PPI" can have quite visible differences in resolution, and the PenTile requires a higher PPI to reach equivalent levels of imperceptibility. The percentage of active light emission area to surrounding support area can also have an effect. It's possible that future quantum dot technologies or micro inorganic LED displays or the like could change this again, requiring a lower PPI for the same effect, but we'll see.

Also FWIW "resolution capability of the eye" necessarily involves distance and variations in the general population's resting focal point (there are a lot more near sighted people nowadays). So the PPI necessary to achieve "Retina quality" will be a function of sub-pixel construction, expected typical usage distance (a phone will tend to be used closer then a PC display which in turn will tend to be closer then a living room TV which in turn is closer then a billboard or public projector type system), and exactly how many standard deviations into the population a maker cares about.