Where it says returned it means if it’s being imported commercially. For example you order such materials by paying and some company ships them to you directly, they get stopped, customs may exercise the option to order them returned to sender via whatever courier they were being sent by. For individuals they’re not going to hold your stuff for you and bother making sure it gets back to you, it gets destroyed. More than likely such a provision mainly applies to other prohibited goods, say good faith attempt at importation of something for commercial purposes which is technically prohibited but valuable may be sent back rather than destroyed.