The page opens with a panel of Sanji and Zoro talking to each other & Franky about how it's important they move fast because the enemy still doesn't know they're on Onigashima. The next panel is Usopp noticing guards that Sanji and Zoro have not noticed yet. He WAS looking through his binoculars, but when he notices the guards he's glancing to the side without them. He's noticing them with his observation haki. The next two panels are the guards like a mile away or whatever drinking booze. They're sitting behind cover and barely notice the existence of ships on the horizon. The next panels, Usopp has already headshot every guard with sleep ammo and knocked them out. The other straw hats say "I thought I saw people there!!" after it would have been too late, but Usopp just blows them off and doesn't even claim credit, saying "Looks like they're asleep! No problem!!" It was VERY casual so I guess that's why no one noticed.
The important thing here is: "No Observation Haki Feats" Usopp noticed, identified, and took out a bunch of guards
before Sanji or Zoro even realized there were guards with line of sight on the crew.