I suppose if one thinks only people Jesus specifically appointed and sent out could cast out demons, insisting that he had to be one of the 70 would make sense. I'm not saying you are doing that. Jesus, when accused of casting out demons by Beelzebub, asked if he cast out demons by Beelzebub, 'by whom do your children cast them out'. Jews had methods of exorcism, and apparently some of them worked. The Jews were astonished at His authority casting out demons. It could have been some of the 70 who cast out demons, but I think it is more likely that some people heard Jesus' message, believed that God sent Him, and cast out demons in His name. Maybe they saw Jesus cast out a demon or one of the apostles casting out demons in His name and imitated it. Btw, if anyone says they don't believe in miracles for today but believes Christians still cast out demons in Jesus' name, he's contradicting himself, since Jesus implied that casting out demons in his name is a sign/miracle