Little dated (2012) but basic material properties of eink has not changed:

https://archive.nytimes.com/www.nytimes.com/interactive/2012...

Eink due to its USP and fundamental characteristics only reflects light and does not transmit (i.e., is not transparent. Transparency is needed for back lighting).

Are you sure it’s a backlight? I’m looking at my Libra 2 and can’t tell whether the light’s in front or in back.

I can say perceptually it's a backlight, how they did it in engineering terms I'm not sure.

It can even be in dark mode and variably backlight only the foreground content with no change in the illumination of the dark background.

Maybe its back but conducted to front through a piece of translucent plastic? I.e. still a frontlight.