'Reflected light is not the same as direct light at a cellular level'
T.H. Chan Harvard School of Public Health
https://sid.onlinelibrary.wiley.com/...1002/jsid.1191
A peer-reviewed study designed to measure the biological stress placed on retinal cells found a quantifiable difference between the technologies. Concluding that "cells accumulated ROS [Reactive Oxidative Species] two to three times more slowly on exposure to frontlit electronic paper displays compared to backlit liquid crystal displays."
This means that even at a comparable brightness, the light from a backlit LCD induces significantly more stress. It found that eink devices can be used three times as long as an LCD before reaching the same level of stress in retinal cells.
The thought of equivalency at the 'same light level' ignores the reality of variable ambient light. A reflective display works in harmony with its environment. It does not compete with ambient light, it uses it as its primary source of illumination. The optional front light supplements this preserving a reflected-light experience unless it is the primary source of light.