And he would have said 'upon You' had he been referring to Peter...But he didn't...
The point here is that He was talking about "THIS" Rock, which would apply to the man he had just named "Rock."
That would have been some pretty bad English, and Greek...Jesus called Peter a rock and Jesus knew well that he himself was the rock spoken of in the scriptures...
Jesus was contrasting the rock of Peter with the rock of Himself...
The whole weight of the rest of Scripture tells us that Jesus/God is the Rock.
The Catholic church took ONE verse of somewhat ambiguous interpretation, and builds a whole doctrine out of it that directly contradicts the rest of that whole weight of Scripture that tells us otherwise.
That smacks of agenda and should give any serious student of God’s word at least a pause to consider if the Catholic claims are legitimate.