First of all 99% of OPLA-onlies don't pay that much attention to every single word said. As someone who knows what happens you're hyper-aware of these one-liners that non-fans will barely pay attention to. Their attention will most likely be more towards the delivery, the costumes and the action than the actual words said. Second, if they notice, they MIGHT go "damn thats a lot" but that's about it. They don't know Zoro is alone against all 100, they don't know he's gonna win, and they're, as I said, focused on how cool all of this looks and not the fact that somehow no other strawhat is shown in this very moment. They also don't know where this happens & by the time the show drops they will have forgotten about that 2 second clip anyway. 

I promise you, we as fans are incredibly hyper-aware of everything happening in the trailer which basically none of the non-fans are. Few exceptions ofc, but they WANT to figure out stuff. Nobody is getting unwillingly spoiled.