If you want the easy way, look for .hex files. They’re precompiled and all you have to do is follow the guide at FW3A Firmware Flashing Guide. You can use the (older) FW3A-219c hex file and it will work well enough: http://toykeeper.net/torches/fsm/anduril2/anduril.2021-12-13.fw3a-219.hex
If you want to go the proper way and compile Anduril2 from source, have a look at the branch anduril2 : Code : Flashlight Firmware Repository for Anduril itself and find the config files mentioned in the Reddit thread you linked in the branch anduril2-fwaa : Code : Flashlight Firmware Repository.
For flashing a Attiny13, have a look at Guide: how to flash ATtiny13a based drivers (NANJG, QLITE, etc.) with custom firmware.