Yes, just like you can perm brick N9 by writing rnd() to CAL partition, if you destroy your bootloader with rnd() values on purpose, then your device is quite a brick (and out of warranty). As even @care uses fastboot to do reflashes, so they wouldn't be able to help you. In case of bricked bootloader, it can only be fixed with non-public factory tools and special factory cable which is only avail in factory-line and few Jolla employees.

So my advice would not mess with bootloader partitions, as those are the only things that you cannot recover from yourself if you happen to accidentally do a mistake. Even I have bricked one device when playing with custom bootloaders (luckily I can recover from accidents)