a christmas carol - juxtaposition??

okay so basically im revising for a christmas carol and watched a video by ''Mr Taylor'' who analysed Marley's character and he states that the quote ''Old Marley was as dead as a doornail'' is 'a juxtaposition of of Marley and metal foreshadowing his later appearance as a door knocker and the chain carried by the ghost' and I got confused because I thought juxtaposition was meant for 2 contrasting ideas and it could be argued that Marley's character was in fact LIKE metal; cold and unfeeling so I don't know, is it juxtaposition? And how is this relevant to the door knocker?