Yeah I don’t disagree. But on the whole I think it gets you a long way towards it without the cons of having two devices for example.
I think that I suffered more eyestrain than usual, mostly because I could not tell icons and other UI elements apart. Yet I don't have this issue on the E-Ink device, interestingly, even though I use many of the same apps e.g. Telegram.
I would say that the difference is due to the reflective vs radiative screen, not the greyscale palette.
So for me, the whole point is I won't be using it enough to notice eye strain or something like that. If I'm messaging or doing something the device is designed for, it's a benefit for me to be able to have it in full fidelity.