I don't think this is a reliable way of checking whether the boot was caused by the RTC or the some other reason. From the BM8563 datasheet (my highlighting):
18.104.22.168 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.