Chu Feng was making a bet. What he bet was Ya Fei only detecting that he sent mental messages to the monster, but, not the things he told the monster.
The reason why Chu Feng made such a wager was because he did have some confidence. Although he knew that Ya Fei was also a World Spiritist and very possibly her Spirit Formation techniques were above his own, Chu Feng felt that even if his mental messages were picked up by Ya Fei, she might not have clearly heard what Chu Feng said.