For example, when we speak of what Paul had, we invoke a probablistic mode of analysis that asks, not what did the Roman See consider important 400 years into the future and work back from that, but what would a typical Jewish Christian apostle of the First Century most likely have in his possession, based on what we know of the Jewish Canon at that point in time. Under that model, second class documents not being treated as canonical at that time would not pass that test. We know Paul had regard for the Jewish magisterium, as he stated to them were given the oracles of God. If they had not accepted the deuteros, and we know they didn't, we would expect Paul to make an argument for their use, and also to use them, as he was very well versed in the Scriptures. But in fact everything he used, as having the force of canonical authority, was from within the limits of the smaller Jewish Canon. This makes sense. Speculating forward 400 years then reading it back to Paul is circular reasoning at its finest, and therefore completely untenable, at least to us evangelicals, who are not ever going to be comfortable with such blatant circularity.
Most diaspora Jews had a difficulty reading Hebrew and read Septuagint instead. That is the reference St. Paul is making: not the scripture "read in synagogues" or "canonized by one or another Jewish body" but scripture "known". No one is denying that the Jewish race gave us Mary and through her Christ, and had the oracles of God prior to the Sacrifice of Christ.