Comment by jeroenhd

Qualcomm phones come with a special mode (https://en.wikipedia.org/wiki/Qualcomm_EDL_mode) that allows devices to get unbricked even after you break the normal user-updatable "bootloader" on flash completely.

This feature doesn't allow unlocking the bootloader (as in, execute a custom ROM), it's designed to install factory-signed code. However, using it to "restore" an old, vulnerable factory code would obviously cause issues.