I might have misread, but it appears he’s stating that moral values in the secular world, held outside of God or a belief in God, cannot be proven. Christians can prove theirs through the Bible. What can secularists use to “prove” moral values?
If your interpretation of his claim is accurate, then the claim itself is correct. (I didn’t read the whole article.)
Without God, there is no objective basis for morals.
If that is what he’s saying, he’s correct.
Secular morals are not based on absolutes. They are based on subjective terms and beliefs, there is no absolute to point to to show/prove any ones’ secular morality is measurably better than any others.