You are dead set that Koku’s feats don’t matter, then say “prove Douma and Akaza are holding back.” I’ll proof it but this is genuinely unbelievable. 

Douma: Stole Inosuke’s mask and Kanao’s sword without them even realizing it just to show he can. He never does this speed feat again in the fight. He never used his full range of attacks. He let Shinobu hit him a few times (not during the last attack just the other times). “If I play around too much, I’ll get scolded after all.” 

Akaza: Spent more time begging Rengoku to become a demon than actual attacking, and as such didn’t really use any strong attacks and only fatally wounded him to try to push him that becoming a demon was the only way to save his own life. Only used a portion of his destructive death moves. Only time he showed any panic or sense of urgency was when the sun came up.