Fact? LOL.
For more than one reason. One, Paul was a Jew, a Pharisee, and knew the Hebrew Scriptures IN Hebrew probably better than any of the other Apostles - he didn't NEED to refer to the Greek. Second, he never mentions any of the Apocryphal books in his epistles either directly or as inspired like he did nearly every one of the 39 actual inspired OT books. He obviously WROTE his letters in Greek, but there is nothing to prove he only had the Septuagint to go by when speaking of the Law and the Prophets. Not to mention, there is NO proof that the Greek Septuagint EVER included the seven books (or the other EIGHT) in any section at all associated with the non-contended OT books that Jews and Christians hold as divinely inspired works. There is even a question of what books even WERE part of the Septuagint, seeing as it began only containing the Pentateuch (first five books of Moses) and went through development over centuries, its history is clouded with legend and myth.
I would think seeing that several of those Apocryphal books overtly testify to NOT being the word of God but thoughts of men, contain outright errors - which God would NOT have made, come right out and admit that there WERE no prophets of the Lord in that intertestament period, it should be more than adequate proof that the Apostle Paul, nor any of the others, would have thought of, much less relied upon such writings for anything.
It would be refreshing to see a Roman Catholic just come right out and admit what we already know that the ONLY reason they argue in favor of those seven books is because their magesterium at Trent declared them to be God-breathed Scripture so that they could settle the question of the canon in the face of the challenges of the Reformation. They're basically stuck with having to defend them - not all that different from many of the other invented dogmas over the years.