It could be the Trinity. It could be the union of Jew and Gentile in Christ. It could be just about anything.
I don't think it's the Trinity, I think he's referring to the way in which actions of God (in the Old Testament) were attributed to Christ. This is clear from his quotation from Psalm 24:7. This sharing in God's glory by his Messiah is something that certainly need not lead to Trinitarian theology, but in the early centuries of the Christian church this is the direction in which some people took it. That doesn't make the concept itself wrong of course, just what was eventually done with it. (And there were other factors that contributed to it as well of course, such as views of the atonement, dualism etc.)
Whatever it is, it doesn't negate the many valid points raised by this article.
I agree with this. In my comment above I was just musing on the difference that a non-Trinitarian view of Christ might make to our attitude to worship.