The "Biblical Trinitarianism" mentioned in the title of this article refers to what has become the standard way of arguing that the Trinity is a Biblical doctrine, which is:
So, there it is - easy. The Trinity, from Scripture, in black and white. Right?
Not quite, for a few reasons. Firstly, the above formulation violates a basic, uncontroversial implication of the law of identity - the transitivity of identity. If a=b, and b=c, then by the law of identity, a=c, that is, a must be c. So, if points #2-4 are true, then points #5-7 are demonstrably false, because if the Father is God (a=x) and Jesus is God (b=x) then the Father just is Jesus (a=b).
Another issue with the formulation is revealed by asking whether God is triune. If God is triune, then points #2-4 necessarily mean that the Father, Son, and Holy Spirit are each triune, also by the transitivity of identity. To affirm "God is triune", and "Jesus is God", means that Jesus either must be triune, or He is not God.
So, points #2-4 just don't work. They need to be rephrased, and qualified, in order to avoid demonstrably contradicting points #5-7, and the proposition that God is triune.
Secondly, points #3-4, that Jesus and the Spirit are God (the formulation supposes God in the absolute sense), are not taught in the Bible in the way required by the formulation, as covered in the article entitled The Biblical Case for Unitarianism - Five Facts. The Bible instead teaches that the Father alone is God (point #2), following the words of Jesus Christ, who said:
John 17:3
3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
Therefore, not only is the common "Biblical Trinitarian" formulation invalid strictly speaking, but it also contradicts clear Biblical teaching, that God just is the Father (1 Corinthians 8:6, Ephesians 4:5-6: 1 Timothy 2:5).
Finally, points #2-7 seem to logically imply that #1 is false. Breaking the doctrine into these points is essentially an attempt to justify believing #2-7, despite not being able to understand how #2-7 fit with #1. Apologists and scholars who use this formulation ostensibly think that if they can prove points #2-7, then they don't have to worry about how they fit with the fact that there is one God, because somehow, it must all work out, so long as #2-7 are proved (#1, that there is one God, is Biblically certain, and so it is taken for granted).
However, attempting to reconcile #2-7 with #1 is where all of the complexity commonly associated with the doctrine of the Trinity comes from. Thinking that points #2-7 are true doesn't make that problem go away. Declaring victory because one thinks they have proven #2-7, and then takes #1 for granted, doesn't work, because it doesn't provide a coherent worldview. It simply is an attempt to Biblically justify believing something that cannot be explained, and does not make sense.
This common formulation is often grouped with what some Unitarians call "the standard opening move", which is for the Trinitarian apologist to say that "The Trinity is not incoherent, because it is not saying that x=3x, but rather, that x=3y. It would only be incoherent if we said that one Being is three Beings, but we clearly say that it is one Being existing in three Persons". Of course, Unitarians already know that that is what Trinitarians are claiming. It is usually after this, that the above formulation is brought out, in an attempt to radically oversimplify what is an extremely complex and difficult to defend doctrine.
The formulation is invalid logically, and Biblically. And, even if one thinks that the Bible teaches all 7 points, they often hand-wave away the fact that point #1 - that there is one God - still doesn't really make any sense in light of affirming points #2-7. Maybe that should indicate that the quest to reconcile them was mistaken from the very beginning, and that the theory as a whole should be re-examined to check whether all of this complexity is actually coming from a mistaken assumption somewhere along the way.