A Concessive proposition, by definition, will appear to contradict the main assertion. But ponder v. 28 awhile. “No one is greater than John” yet “even the least in the kingdom is greater than John.”
The starkness of the Concessive in v.28 highlights the Progression to this verse as the Main Point of Jesus’ message (v.24–28):
Do not doubt John’s integrity. He is the greatest of the prophets. Nevertheless, as his whole ministry was only to point to Christ, even the least among those who believe in Christ is greater than John.
This may run counter to all human expectations, but God’s wisdom will be proven out in the end.