Agreed, but that is not what is at issue. Marcion's error was to claim that the God whom the Jews worshipped (i.e. the God of the Old Testament) was not the God whom Christians worshipped. The issue had nothing to do with whether the Jewish denial of Jesus's divinity was incompatible with the Church's proclamation of Jesus's divinity. Everyone agreed about that. If you want to say that differences in *conception* of God necessarily entail differences in *reference*, then I cannot see how you avoid Marcionism, because the Jews of the OT era had a different conception of God than did the Christians of the NT era. And that would entail that Jews of the OT era worshipped a different being than than the being worshipped by Christians of the NT era. And that is Marcionism.
-A8
Easy; God had not been revealed in full until the New Testament. After God had been revealed in full, sure the Jews by ignoring this and maintaining an image of God which is 1/3rd at best of the truth do not worship the same God.