does anyone know how would e-ink compare to oldschool reflective TN LCD displays (those in Casios from the nineties)? I have a Playdate device with this type of screen and it seems pretty cool, I wonder why so few devices today are taking advantage of it.

Transflective LCD screens ("e-paper") compete with e-ink currently.

Monochrome e-ink has a better resolution and contrast ratio than old-school LCD devices (I'm comparing my experiences with a Palm Pilot in the 1990s and an Onyx BOOX in the 2020s). LCD can refresh far faster, in the 100+ / 100s Hz range, where typical e-ink refresh rates in my experience have been in the single-digit to low-double-digit Hz range (video is doable but far from ideal).

E-ink also displays quite nicely with a "frontlight", which brightens the background (whiter whites) without washing out the foreground (print/ink). Illuminated LCD displays tend to wash out the dark fields, though I've not viewed e-paper directly and cannot speak to that.

TFA is describing a far higher e-ink refresh rate than I've experienced directly.

I also have a Playdate! I think it's a Sharp MIP rather than TN LCD. MIP is actually pretty popular in some places -- particularly smartwatches where battery life matters more than bright colors; Garmin, Coros, Pebble etc. all use MIP displays for the lower end models.

The thing about MIP is that the viewing angles are just not that amazing. I have had a Kindle and a Kobo, and they look like paper no matter how I hold them. My Playdate however needs to be positioned at a pretty specific angle with respec to the light to get the best contrast.