usually CMOS chips have input protection diodes which prevent the input pins from rising too high above the power lines or too far below ground, which causes SCR latchup - essentially a short from power to ground. This also means that CMOS chips can often be powered by input pins alone.

I've even experienced this with a stepper motor driver which didn't have the main power lines hooked up (but did have logic power), where 3A were going through these diodes to run the motor. So I'm not sure how that particular anecdote says much about the power usage of the ARM chip.