Him saying the D-word is not a momentary, spur-of-the-moment thing. Shen Yuan meant it when he uttered it— and it came after making the abacus inside his head incessantly clatter. He thought of it so deeply that he even overlooked the darkening of Bai Tingfeng's expression.
This incident with Zhou Kai was an eye-opener for him because he realized that he might be unintentionally hogging the limelight that was supposedly meant for the handsome male lead. Maybe this isn't just about him being an omega. Maybe there is more to this picture that has yet to be unveiled.
If he were just the original Shen Yuan, who is nothing but a beautiful flower vase with negligible B-Class power, then those eyeing Bai Tingfeng for all kinds of reasons wouldn't spare to look at him. However, this changed when he flexed his powers during the incident and showed a force equal to that of an S-Class awakener.