The Cortex-M4 is the goto low-power CPU.
However, @rurban is probably correct purely because Ambiq is building their portfolio around the M4 (and M33), and they seem to be working hardest in pursuing and patenting IoT layouts/technologies that reduce energy usage.
I love AVR for teaching/learning, but the fact that you can* get a more powerful, larger memory Cortex-M at a much lower cost than, say, an Atmega... I don't know why anyone would choose AVR for a new design outside of niche uses.
(* - pre-2020)
i have no idea whether there'a just scant dew wins to be had, or whether this complete lack of innovation is a company asleep at the wheel.
do you also love the cortex-a53 for application processing? because it too is another dinosaur relic that we seemingly cant eacape from. a55 seems to have made a like 12% difference.