No, he's trying to do a buster99 by wiping all partitions and then flashing the Asus firmware blob. If you get an 'unrecoverable bootloader error' that's pretty much your only option. The commands are ok but something is off with his fastboot setup - no idea what though. Unfortunately fastboot error reports are so general, they are useless.
I would try it in Linux, that would at least exclude all the issues Windows has with fastboot/adb. Maybe set up an old PC with Linux Mint or try a dual boot alongside Windows. Or even better, ditch Windows for good and run Linux :victory:
Sent from my Pixel 2 using Tapatalk
Wont flashing the Asus firmware take him back to the Asus recovery console also? - NP