Why didn't it work before? That's because busybox expects you will be using their init with their shutdown and reboot. The utilities only signal init about the need to shutdown or reboot.

This hardshutdown is based on the one found in the busybox tree, but enhanced to use the RESTART2 type of syscall, allowing "reasons" to be given for the reboot.

In turn, those "reasons" are used by the platform, here I'm assuming an android-based device, to "do stuff". With most (all?) android devices you can directly reboot to bootloader using this "reason" by using reboot bootloader.

It may be possible to reboot to recovery using reboot recovery, but this has not been tested successfully on two devices.

Note: the reboot command from systemd already supports passing a reason to the syscall, so from stage-2 it already was possible to do this.