No, since fastboot mode is provided by the littlekernel-based bootloader loaded by the SSD. If the current boot slot bootloader is broken and the device isn't in a post-update mode where it automatically rolls back to the other slot then the device is bricked. This can't generally happen when the device is locked. An unlocked device is a lot less reliable even without using fastboot mode directly. Verified boot helps it avoid getting bricked in multiple ways.