The Omrides flirted with polytheism certainly as the Bible makes clear, though I'm not certain where he gets the idea that it was Israel's greatest extent.
New archeology, theory attributed to archeologist Israel Finkelstein if I remember correctly.
States that Israel was actually majority polytheistic for the most of its existence, with Yahwe being the premier god of the Canaanite pantheon. A couple of ancient Kings attempted to institute a cult of Yahwe, modern Judaism as we know it, and they were credited with the extent of David and Solomon's kingdom in the bible, when in actuality the large kingdom that was said to be David and Solomon's was really the polthyeistic Omrides kingdom. Attributed to David and Solomon for numerous reasons, mainly to give the Yahweh Cult (Judaism) historical precedence and justification. This archeology also shows Jerusalem was nothing more than a small village during the actual time of the Kings that represent David and Solomon.