It's not so much a trinitarian thing as an alternate understanding. After the fall of man, the Father, El Elyon, could no longer deal with man in his sinful condition. He chose his Son, Yahweh, in the beginning, to be the mediator between God (meaning the Father) and Adam (Man). Yahweh was the Son, the Messiah, Redeemer, the stone and God of Israel, Jehovah. He was the God of Abraham and Isaac, the one who called and spoke to Moses, who led the children of Israel from Egypt into the land of Canaan. Then, in the meridian of time, he was born into mortality and raised as Jesus, suffering and dying for mankind. At the end of the Millennium, he will present the Earth, sanctified and redeemed, back into the hand of the Father. It doesn't require the Father and the Son to be the same entity. They are ONE in the same sense that the apostles were to be ONE.
If one reads the NT, frequently the writers make a distinction between God and Jesus. They can indeed make a case that Jesus is not part of God. Even so, the apostle John seems to close the gap on that, especially in Revelation. In his writings, Jesus sits down with the Father on His throne, as we sit down with Jesus on His throne (which is the Father's throne). Thus, we become like Jesus and Jesus becomes like the Father and we, Jesus and the Father all become ONE. Since Yahweh is called God in the scriptures, and is later identified as Christ, we can conclude that Jesus (Yahweh) is a different being associated with God.