It makes me what other things could be much better if they received some attention from someone who cares about it.
Is one a subset of the other? If so which is the superset?
E-paper is an informal category that anyone can use for any display type.
The display in this video/article is also electrophoretic, but not all displays called e-paper are and it’s aggravating. An example: https://kentdisplays.com/products/color-epaper-displays/
"E-paper" means very little. It may mean "static" (and yet not necessarily "bistable", not even necessarily "efficient"), it may mean "reflective"... It is a marketing term.
So no, there is no real "agreed understanding" in it.
Some 15 to 20 years ago there were a lot of other companies trying to create reflective (= e-paper) screen technology based on principles other than electrophoresis, some with native color or fast pixel response times, but they all closed shop.
The only other major type of e-paper that is still used is simple reflective greyscale LCDs. These are not bi-stable but achieve fast pixel response times. The main disadvantage is that they have significantly worse reflectivity, resulting in rather greyish whites.
For the grayscale, I'm guessing each physical display would need to be individually calibrated? I also wonder whether the calibration is sensitive to environmental factors, i.e. gets out of whack if the display is too hot or too cold.
There has to be a reason the e-ink industry hasn't done this yet...
Reading a book on e-paper in daylight is so much nicer than on a phone, for example.
Is also "reflective" (natural) vs "emitting" (a compromise idea that worked but should not have you take it for granted - we do not "read fire" in nature).
LCDs are inherently transmissive rather than reflective (like the electrophoresis-based screens from E Ink) or emissive (like CRTs or OLED screens). To make them reflective, the ambient light has to go through the liquid crystals and a polarization filter twice, since it is only reflected by a mirror surface behind the screen.
Unfortunately they are not strongly transmissive, so they absorb a lot of light, especially when it has to pass through twice. This is not a problem for LCDs with emissive LED backlights, because the light has to pass through only once, and LEDs are highly power efficient anyway, so you can just make them brighter. But you can't simply dial up the ambient light on reflective screens.
They're great if monochrome IMO, but the color variants suffer with brightness as each subpixel can only reflect 1/3 of the light, which makes the backlight necessary in more situations.
Another issue I heard, but I don't know enough about the physics of e-ink to know if this is correct, is that if you are at a gray level, the white and black particles are mixed and might exchange charge, thereby causing some particles to not move correctly anymore.
Great stuff.