I don't think that's true as long as you provide healthy lightning. Light is light, doesn't matter whether it came from the sun or from backlight lamp. E-ink obviously is preferable when you're reading in the direct sun light, because LCD could be barely visible in those conditions.
One significant difference is that with e-ink you can provide healthy lighting. With an LCD display you are stuck with whatever backlighting is built in.
Light from the sun contains a much greater number of wavelengths (and different depending on time of day) than that from the backlight; it's not the same.
