Forum:Red dragon archfiend + your own defence monster

i've been using red dragons archfiend for one of my backup powerhouses in case my stardust fails. but one thing that confuses me is what i have been told by two different people regarding its second effect of destroying any other monster you control that did not attack. one person told me using savage coliseum is that if a monster is in defense mode, they are not destroyed by the card effect due to the fact they can not technically declare an attack, but yesterday when i was facing an opponent. they said that the defense position monsters i control are destroyed because they are in defense position. so the question is, if i have red dragons archfiend on the field and two set monsters on the field. are they destroyed at the end phase