The market they cater to are those that want to use linux distros in devices beyond PCs and laptops. For that market, the only eink tablets that exist are this

The market they cater to are those that want to use linux distros in devices beyond PCs and laptops. For that market, the only eink tablets that exist are this one, to a lesser extent reMarkable (but AFAIK you can't really run X11 or Wayland on it), and maybe an eink portable monitor with an RPi or similar board and a battery bank taped to the back. Onyx Boox doesn't qualify. From that perspective, with such few choices of tablets, $400 doesn't seem that bad.