The page you linked doesn't mention PPI or DPI at all. From what I can see from a quick search, viewing low-res images strains the eyes but I'm not sure how that relates to pure text.

It also specifically mentions glare and reflection being part of the problem, something eInk displays are better at than normal displays IMHO, because they don't project light at your eyes.

Do you have any links to "the science" here? Because a quick search seems to pull up a few things supporting less eyestrain from eInk than other screens, regardless of resolution. For example - https://pubmed.ncbi.nlm.nih.gov/24386252/