i don’t think that noting that the prevailing Jewish idea of God is completely incompatible with the Christian one could be described as Marcionism.
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