They are Jews, as much as anyone else. There are Messianic believers right IN Jerusalem. King David was a Messianic Jew. HE is the one who prayed and asked for the Son of God and the helper(The Holy Spirit -ruach ha-kodesh ) !
Many Jews believe in Jesus. Some Jews are in Christian congregations. Some Jews are Dutch Reformed, evangelical, Baptist!
If you are offended, that is about you, not about what is being written.
A Jew who follows another religion is Jewish only to the point that he still has a spiritual obligation to repent and to return to Judaism. Belief in J.C. and claiming to be a Jew are antithetical. You cannot be both, except perhaps in your own mind. I know they would not be welcome in the Jewish community.