> Jinbei suggest that Sanji may have sensed someone strong, but we know it's wrong since he didn't go due to sensing someone strong, but rather heard the sound of a female in danger
So Luffy saying that his observation haki musn't be good enough then is supposed to be a sarcasm considering Jinbei's supposition is wrong. It's just that no one in the roof is strong enough to be worth considering for Luffy