I need to wear glasses with my retina display, but not with my e-ink reader. The eyes are definitely doing more work when looking at a backlit display for whatever reason.
also, same color scheme? I've seen some studies that show it's harder for people with certain vision issues to focus on text that's white text on a black background than it is for them to focus on black text on a white background, so.. if they're trying to read on a dark theme on their phone, that could be noticeably different from using their E-Ink screen, which is almost certainly using a light theme.