The idea of a bi unity of Jesus and God comes out of the old testament and the generally idea what new testament writers believed about Jesus. I don't feel like writing another essay about it but it's generally pretty simple.
It's not some Devine revelation bullshit like the watchtower teaches but a matter of understanding old testaments quotes and ideas used in the new testament.
I really don't know why they think they have such a slam dunk on the subject.