> The GPU is still in charge

Can you explain in specific, concrete terms what this means?

For example: I've read that the undervoltage monitoring happens on the ThreadX process, and that it throttles the ARM cores when low voltage is detected. The vcgencmd has more info on this, including the following bit field returned by vcgencmd get_throttled:

    Bit Hex value   Meaning
    0          1    Under-voltage detected
    1          2    Arm frequency capped
    2          4    Currently throttled
    3          8    Soft temperature limit active
    16     10000    Under-voltage has occurred
    17     20000    Arm frequency capping has occurred
    18     40000    Throttling has occurred
    19     80000    Soft temperature limit has occurred