Note that it does not appear to have an e-ink display. They called it e-paper which doesn't necessarily mean the same thing.

The display appears to be this one: http://www.sharpmemorylcd.com/1-26-inch-memory-lcd.html or something like it.

It's transflective so you can read it outside, and inside you can use a backlight (which they list on the tech specs). It's very low power, so you can leave it on all the time. Those two things combined are why they call it e-paper.

Personally I think it is a bit confusing on their part, but I understand them wanting to get across that it's always on and visible outside. However, it does get confused with e-ink.

Their other watch (inPulse) uses an OLED display that looks similar.