The Twelve only received the mysteries of the kingdom according to Matthew 13:11. That is totally different from the mystery that Paul received that had been kept secret since the world began (Romans 16:25; Colossians 1:26).
Notice the parenthetical (as I wrote afore in few words). He is most likely referring to Ephesians 1:9 and Ephesians 2:19, but, he could have also been looking back to Galatians 1:15-16. There are those who hold what is called an Acts 28 position which says that Paul did not have the mystery revealed to him during the Acts Period. Therefore, anything he wrote during that period did not contain it, i.e., Romans, Corinthians, Galatians, and Thessalonians. Therefore, only Ephesians, Colossians, Philippians, Philemon, Timothy, and Titus apply to the Body of Christ. Those who hold this position say that Paul preached the Kingdom Gospel in the Acts Period and the Grace Gospel after the Acts Period.
Verse 5: Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; Again, Paul is clearly saying that what he received was not what was given to the Twelve, or anyone else for that matter. And now it has been revealed through Paul to his holy apostles and prophets by the Spirit (Ephesians 2:20).
Some commentators will refer to Peter at this point as having received the same revelation back in Acts 10:9-20 before going to Cornelius' house. Sure, Peter's vision was that the Gentiles were in, but that was not the same revelation that Paul had received that they would do so outside of the law and completely by grace. We know this because Peter preached the kingdom gospel to Cornelius and continued to do so for the rest of his life. Unlike the grace gospel, the kingdom gospel required baptism, repentance, and adherence to the law. They were not the same. "Things that are similar are not the same."