Everybody knows that already even before the cooking fest started.
What I'm thinking is Kousairou could be the double agent. I'm still puzzled with what he did to Setsuno. I think that when he used the calorie stealing technique on her, he may have just stolen a few amount of calorie and not 10 million kilotons.
Because that same technique beat Chiyo easily but it seems that it did nothing to Setsuno.
Setsuno may be stronger than Chiyo but I don't think it's to a level that the same technique that 1 shotted Chiyo would be effectless on Setsuno.