> Moses and David were not champions of Christianity
I disagree. Jesus is the fulfillment of God’s covenant. However, the Old Testament heroes were aware of that covenant, though obviously they did not know what form it would take at the time, though the prophets foretold how it might. Their faith was rendered unto them as righteousness before God. That they sit with God today means that they championed a faith that God would fulfill His promise. And He did through Jesus Christ.
You are free to disagree, but you and the author would still be wrong.