he says this because at the moment, he was being scarcastic
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Roger and Maurice are the ones that attack Jack and Piggy because Jack wants Piggy's glasses.
After Jack leaves to go to Castle Rock, Piggy becomes more confident with himself, and starts to stand up for himself. This happens because Jack stole Piggy's glasses, and now he can no longer hide behind his disability of not being able to see.
Piggy
Piggy is fully aware that Ralph is the only thing that is preventing Jack from becoming chief. Piggy knows that if Jack does become chief all hope of rescue will disappear because Jack will only bother about hunting and the signal fire wil be forgotten. Piggy is also fearful for his own safety.
Well in chapter 10 Jack and his tribe of savages invade Ralph's tribe (made of up Ralph, Sam and Eric, Piggy, and one/two littluns) in order to steal Piggy's glasses, which is overall the power to start a fire on the island. Jack might have mentioned this plan in chapter 9 so I'm hoping this is the right answer to your question.