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.