If that were true, GarminCure3 would literally kill devices with 100% reliability, as there would be no other ways to update the firmware.
But no, pre-boot mode has to work, because bootloader inside boot.bin is not patched.
You can flash some garbage instead of fw_all.bin and pre-boot mode would still work on 99% of Garmin devices (on GVN53 it won't, sadly). This proves that pre-boot is not implemented in fw_all.bin. And GarminCure3 patches only fw_al.bin.
The answer seems to be too obvious... Maybe I didn't understand you.