The only Scripture that existed at that time was the Scripture that Yehoshua/Jesus used. The Tanach/Old Testament. Everything else has been ADDED and the Old Testament warned about ADDING to it.
Ok, so you don’t believe Paul was speaking authoritatively for God, and I do.
If you believe the OT, then, certainly you believe that we are all descended from Adam, so we are all “one blood” in the sense that Noah Webster construed that to mean.
Ergo, Paul was speaking correctly in saying what he did, basing it, as he did, on the Creation Story to which he explicitly referred in Acts 17:24-25.
Therefore, I submit that Paul was speaking authoritatively, basing his assertions on the Creation Story in Genesis.