Holding back might be the wrong term here, not going all out is what I meant.
Round 1 vs Luffy he trolled Snakeman and stalemated/outlasted G5 with his mission not about Luffy. Round 2 was him mentally nerfed from killing VP he was a shell of himself there, was probably looking for an out if anything.
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BM could not even perceive Brook.
She always relies on homies to tag opponents, she's not tagging Kizaru without them, since they don't have haki she's cooked.
Also Kizaru can butcher BM with a light saber, everything points to lasers being effective against her too, but that's not really relevant to the discussion, Kizaru can easily pull up on Kaidou and BM and leave whenever he wants without them being able to do a thing about it.