It is so simple, that your suspicions regard it betray more your fear that
Mcn precedes all the Gospels than a real skepticism about who is of Tertullian and what is not (but this is - obviously - only a my personal suspicion).
At any rate:
My present discussion is, how the evil spirit could have known that He was called by such a name, when there had never at any time been uttered about Him a single prophecy by a god who was unknown, and up to that time silent, of whom it was not possible for Him to be attested as the Holy One, as (of a god) unknown even to his own Creator. What similar event could he then have published of a new deity, whereby he might betoken for the holy one of the rival god? Simply that he went into the synagogue, and did nothing even in word against the Creator? As therefore he could not by any means acknowledge him, whom he was ignorant of, to be Jesus and the Holy One of God; so did he acknowledge Him whom he knew (to be both). For he remembered how that the prophet had prophesied of the Holy One of God, and how that God's name of Jesus was in the son of Nun.
http://www.newadvent.org/fathers/03124.htm
Here Tertullian asks: if in virtue of the same logic of Marcion (the fact that even the demons couldn't recognize the alien Jesus, so their recognition of him as Son of God is necessarily
wrong: viz, they are alluding to the
demiurge as father of Jesus) no entity of this world knew
in advance the identity of the alien, then which
evidence did they have
also only in order to call him 'Son of God' (even assuming that the 'god' here is the demiurge) ?
Hence it is strongly expected that Tertullian has to deny
with great and obssessive insistence the fact that the 'teaching as one with authority' (fact that precedes the false recognition of Jesus as Jewish Christ by demons) has to be considered a great miracle (able alone to raise great wonder by the demons about the identity of Jesus).
Simply that he went into the synagogue, and did nothing even in word against the Creator?
So we are moved to accept, for the Marcion's Gospel, just the precise point that is denied by that rethorical question of Tertullian: the 'new' doctrine given by Jesus
'with authority' in the synagogue is
per se necessarily the previous miracle that scandalized the demons, at the point to make them believe that Jesus
could only be the Jewish Messiah (in absence of any other god of their knowledge).
So the demons are scandalized by the 'authority' of Jesus (believing him wrongly the
Jewish Christ), ignoring that Jesus was dissolving rather the Law of the Creator.