Many critical scholars read Isaiah 9:6 as a royal accession oracle about a Davidic king, with throne names in Near Eastern court style. On that reading it isn’t a prophecy of a divine child at all.

The critical reading is worth knowing, and two replies apply. The claim here is narrow, that the title is not a lesser rank of deity, which holds whoever the oracle originally addressed. And the New Testament and the Jewish targum both read the passage messianically, so the question of its ultimate referent is separate from the meaning of the title. The Witnesses, who accept it as a prophecy of Jesus, are the audience for this argument, and for them the critical reading is not available.