It may be true that the books of the Bible were not written as a theological summary, but when they were canonized by the early Christian church that was exactly their intention. These are the books that were chosen as representative (summary) of the word of God (theos) and it was necessary for Christians beig converted/baptized to accept this "word of God".
It can be said that the Biblical canon contains all truths necessary for salvation. However I think it would be innaccurate to speak of it containing all theological truths. The book was intended as a written deposit of the faith, but not as a self-contained, self sufficient one. The Church itself that canonized the list of books was the full deposit, and the book is an emanation from that source written by members of that body over a very long time. The book is about the revelation of God to man through history, but it is not itself the revelation.
Some truths are directly stated in Scripture, for example, Genesis 1:1. Other truths are not stated directly in Scripture but are implied clearly by the biblical author. For example the personhood of the holy spirit is never stated, but it is implied in texts such as Acts 13:2. Some truths can be inferred from Scripture even though the biblical authors did not clearly imply them. Various biblical passages state or imply that Jesus is true God and true man, but in none does the biblical author state or imply that he had two wills. We have to figure that out by inference. A truth is sometimes alluded to or reflected in the text even though it can’t be proved from the text alone. Teachings held by hundreds of millions of Christians are not found directly in Scripture, but they are alluded to. Some truths are presupposed by Scripture. For example when Paul wrote on how to celebrate as a Church on Sundays, he did not give a detailed liturgy in those passages, it was assumed that the practice was already known by his target audience and that only an adjustment was needed. Or when James wrote about confessing our sins openly. Lastly, there may be some truths that do not exist in Scripture at all.
These same truths, not being obvious from scripture alone were being held as true by members involved in stating what the canon consisted of those many centuries ago. So no, I do not think the intention of the canonization of Scripture was to create a summa theologica. This was left to others.
Burn