Wake on interrupt with aggressive gating and poweredown has been around for about three decades now. Pretty much anything with a battery and an MCU has multiple

Wake on interrupt with aggressive gating and poweredown has been around for about three decades now. Pretty much anything with a battery and an MCU has multiple levels of clock gating / power gating at the core, package, and platform. It's implied when building a low-power device, so I'm sure it wasn't a kindle oversight. (FYI "phantom power" is unrelated, it is power supplied to microphones to power the condenser.)