> It looks like they're memory in pixel (MIP) displays, which are basically reflective LCDs I think.

Reflective LCDs with embedded memory, hence the name. Normal LCDs need to be refreshed continuously, but MIP LCDs remember the last frame and efficiently refresh themselves, so the CPU is free to go into deep sleep as long as the display is static.

I'm pretty sure they were transreflective LCDs with an FPGA to handle the low-power refresh.