22 They were amazed at his teaching, for he was teaching them as one who had authority and not as the scribes.
At this point in the narrative, Mark's readers would know only that Jesus's teaching pertained to the advent of the kingdom of God (1:15). Some interpreters thus have proposed that Mark expected his readers already to know something more extensive about Jesus's message, conceivably from their having read the lost Gospel. Be that as it may, the Evangelist explains why the auditors were amazed: "he was teaching them as one who had authority and not as the scribes". Surely this explanation is secondary.
(From the Earliest Gospel (Q+) to the Gospel of Mark, p. 133, my bold)
In *Ev, the readers don't know even that Jesus's teaching pertained to the advent of the kingdom of God, since the amazement is provoked uniquely by the first exorcism by Jesus.
Mark places the amazement before that Jesus made the exorcism, hence Mark is obliged to have the amazement provoked by a teaching (and one that was not there!), not more by a previous miracle!