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.
Notice that that changes the original principle in question. The original principle in question is that if there is a difference in *conception* of God, then there is a difference in *reference*. Now, to avoid Marcionism, you have added the "ignoring" factor. So the revised principle would be something like this: "If there is a difference in *conception* of God, and if the person holding a false conception of God has been presented with the true conception of God and yet has ignored it, then there is a difference in *reference*.
So imagine an old Jewish man who has never heard the Christian gospel, but has worshipped God his whole life according to the OT. He then hears the Christians gospel and rejects it. According to your revised principle, his rejection of the gospel would entail that as soon as he goes back to the synagogue and begins to worship what he thinks is the same Being he has been worshipping his entire life, he has instead suddenly begun to worship an idol (i.e. some being other than God).
Is that really plausible?
-A8