Paul was not present at the Ascension of Jesus. He saw the Lord in His glory, as did Stephen, but he wasn't referring to the actual event as an eyewitness. The whole point is, just because the Assumption wasn't written about by those who discovered her body was gone doesn't mean it didn't happen. Again, the implication is that everyone at Chalcedon was a liar and a heretic. Further, given that the Assumption of Mary was not an issue challenged by schismatics of the day, there's no logical reason they would just make it up out of whole cloth.
"Paul was not present at the Ascension of Jesus."
No, but he saw and had an audience with the risen Christ in person, while he was an enemy of Christ and his life was changrd from enemy to follower because of the meeting. He was an apostle born out of time and none of the other apostles have disputed his testimony to the meeting and in fact Peter affirms his writings as truth. While the actual witnesses gave Luke the account of the ascension, Paul confirms the ascension with his account of meeting the risen Christ and further testifies to the truth of what Stephen saw since he would be the only reliable witness to give testimony of it to Luke, since he was there.