This does not refute Calvinism. Only those whom the Holy Spirit regenerate will honestly confess him before men. Remember, at that time, to confess Christ had a more serious consequence than it does in the US today (think modern-day Iran or Indonesia).
Let's take a look at Romans 11 passage:
Romans 11:17 But if some of the branches were broken off, and you, being a wild olive, were grafted in among them and became partaker with them of the rich root of the olive tree,
Romans 11:18 do not be arrogant toward the branches; but if you are arrogant, remember that it is not you who supports the root, but the root supports you.
Romans 11:19 You will say then, "Branches were broken off so that I might be grafted in."
Romans 11:20 Quite right, they were broken off for their unbelief, but you stand by your faith. Do not be conceited, but fear;
Romans 11:21 for if God did not spare the natural branches, He will not spare you, either.
Romans 11:22 Behold then the kindness and severity of God; to those who fell, severity, but to you, God's kindness, if you continue in His kindness; otherwise you also will be cut off.
Romans 11:23 And they also, if they do not continue in their unbelief, will be grafted in, for God is able to graft them in again.
Romans 11:24 For if you were cut off from what is by nature a wild olive tree, and were grafted contrary to nature into a cultivated olive tree, how much more will these who are the natural branches be grafted into their own olive tree?
Romans 11:25 For I do not want you, brethren, to be uninformed of this mystery—so that you will not be wise in your own estimation—that a partial hardening has happened to Israel until the fullness of the Gentiles has come in;
Romans 11:26 and so all Israel will be saved; just as it is written, "THE DELIVERER WILL COME FROM ZION, HE WILL REMOVE UNGODLINESS FROM JACOB."
Romans 11:27 "THIS IS MY COVENANT WITH THEM, WHEN I TAKE AWAY THEIR SINS."
Romans 11:28 From the standpoint of the gospel they are enemies for your sake, but from the standpoint of God's choice they are beloved for the sake of the fathers;
Romans 11:29 for the gifts and the calling of God are irrevocable.
Now, who does the breaking and the grafting? It is God.
Why does he break off the branches? Because of their unbelief, which is their natural state.
Can the branches that are broken off graft themselves back into the root? No, God does, if they do not continue in their unbelief.
Why do they not continue in their unbelief? The passage does not say. Some will say "free will", others say "predestination".
Christ said “whosoever”, I like Christ, you like Calvin, so be it.