The constant need to prove himself to those around him was a pit-hole factor Chiron saw clear as day in Ralph's life.
The reason the boy always wanted to do it was probably because he felt a standard or expectation for an A-grade user must be met.
Chiron saw this reason to prove himself as an exploitation.
And so he fed Ralphs ego with words he did not want to hear.
If it had been just Chiron saying it to him in a private space, it would have had no effect whatsoever.
However, the time and the place were without a doubt timed perfectly.
Delivering maximum damage to a reputation Ralph had worked so hard to maintain.
Besides, Chiron knew that the worse place to disrespect a man, was firstly, in front of his woman.
No man wanted to be called weak in from of his woman, and secondly, it was in front of people that he considered himself to be better than.
Naturally he was going to prove otherwise.