i dont get it, why the reddit translator automatically assumed the sentence is in plural? Besides in this context, they talked about preparing maximum effort while heading upstairs towards Kaido. It is unlikely they meant it as Beast Pirates as a whole, or even counting Big Mom who clearly wasnt their target atm nor they know/care if she was anywhere near the top of the dome. Their focus was on Kaido.
敵は世界最強の海賊 Teki wa Sekai Saikyō no Kaizoku
(Our enemy is World's Strongest Pirate).
But lets just wait for the official, I just translated the literal meaning of the sentence.