@siscon even without this chapter, the trial arc should give some hint. I know technically Horikita is the one who tell him to do it, but that role can't be just given to anyone. Even after everyone else given up already, he's already preparing for it and starting to push Horikita to the plan. He can concluded what happened so quickly, the three boys should've told him that they encounter Kiyokita(+Ichinose) there, and figured out that Kiyokita at least have a big part there(he already knows Horikita is the Honest type as in she think herself as the better person, and won't think such underhanded plan unless pushed into it. Ichinose is just straight up a Honor student).
Idk how trustworthy Ibuki is, but since putting her there is basically turned back on them, shouldn't he consider even the mastermind deceived her?
Idk man, maybe I just thinking too much about this, but I thought he'd be an interesting character and expected he wasn't that stupid when all the clue just thrown around(since you said he only figured it out later, I guess Kiyokata will outsmarts him at least 1/2 more arcs until he realises it).