This is most likely imo. He might even have known who were in Luffy's crew and who weren't from the newspapers, but most likely he just skipped the people close to Hachi, which means he aimed the haki at everyone except the slaves, pappag, Hacchi and the Straw Hat crew.Actually no.... he said that, while looking to kidd´s and Law´s crew...
its possible that he only didnt targeted the SH because they were the ones closer to hacchi, and because he seemed to know that they were the ones who saved the fishman (he even thanked them for that).
Not that I don't think Usopp could take the Haki.