I have a bad left eye. I haven’t noticed increased readability from higher dpi screens. I went from a higher dpi to a 14” 1080 screen and notice no difference at equivalent font size. Of course that’s still pretty high pixel density.
Higher pixel density can help a bit if your vision is bad enough that you use a lot of screen magnification, as it means that the magnified text will be better-rounded and fuller, rather than almost unrecognizably pixelated.
For some visual impairments, though, the improved sharpness from higher DPI may be basically undetectable, which sounds like what you're reporting.
