Rokudou underwent plenty of character development. Each flashback of past Rokudou, who simply let everything happen and was powerless to do anything, is followed by present Rokudou trying to do something about Hyuma in the present - trying to reason with him, appeal to his character, deescalate the situation. After he gets beat up by Hyuma, he gets up immediately no worse for wear - that's pretty much the direct result of his martial arts training; so he would be able to tell Ranna "I'm not hurt, see?" if something like this exact situation occurs. I don't even think he's putting up a brave face. After all the training Rokudou went through the punches and kicks of an amateur like Hyuma probably don't even register. It's like getting beat up by an angry five-year-old. It wouldn't occur to you hit back against a child.
Old Rokudo was a pacifist because he was powerless and a coward. Current Rokudou is pacifist because of his convictions, and he willingly interposes himself to prevent violence however he can. That's what the chapter is designed to show us.
The chapter also shows us that even with all of this, he still hasn't found a surefire way to handle Ranna. If and when he does the manga ends, so it's the biggest challenge for the mangaka to solve in a satisfying way. But still. No character development? Ya'll need a new prescription.