Which watch has an e-ink(/epaper) display? I can only think of Watchy. Pebble was LCD.
Edit: Sorry, looking further down I see that they say epaper, which is not the same: https://news.ycombinator.com/item?id=42845508
It looks like they're memory in pixel (MIP) displays, which are basically reflective LCDs I think.
Reflective LCDs with embedded memory, hence the name. Normal LCDs need to be refreshed continuously, but MIP LCDs remember the last frame and efficiently refresh themselves, so the CPU is free to go into deep sleep as long as the display is static.
"black and white memory LCD using an ultra low-power "transflective LCD" manufactured by Sharp"