well, there is an alternative method you could use. The power button also acts as regular button. So you could check if the power button was still pressed when M5Paper starts running. Something like this at the beginning of setup():

  M5.begin(true, false, true, true, true);
  M5.RTC.begin();

  M5.update();
  bool pwrBtnStart = M5.BtnP.isPressed(); // true if started by PWR button

Note: for it to work you'll need to press and hold the power button long enough for the code to reach the statement checking the button. In my tests I need to hold the power button for about 5 seconds.

Note: a disadvantage of the power button check method is that when M5Paper is powered on by USB it thinks it was an RTC start.

Thanks
Felix

My practical goal is to have the M5Paper go online and retrieve the NTP time data on a button power on but, in order to save battery charge to simply use the RTC for time data on an RTC restart.

they are overwritten (and cleared) by calling M5.RTC.begin();.

Thanks
Felix

8.3.2.1 Interrupt output
Bits TF and AF: When an alarm occurs, AF is set to logic 1. Similarly, at the end of a
timer countdown, TF is set to logic 1. These bits maintain their value until overwritten
using the interface. If both timer and alarm interrupts are required in the application, the
source of the interrupt can be determined by reading these bits. To prevent one flag being
overwritten while clearing another, a logic AND is performed during a write access.

try to check the timer interrupt enable bit as well, like this:

if ((data & 0b00000101) == 0b00000101) 

Thanks
Felix