When Luffy said it like that, it almost sort of made sense in a weird Luffy kind of way. 'Just' run or fight past all the nurses and orderlies, make it through the doors and up the stairs, then figure out how someone even got on the roof in the first place--he didn't remember seeing any extra stairs on his map. Usopp sighed haplessly and sat up, though when Luffy started counting his eyes snapped up and he gawked a bit.

"Wait wait wait don't count so fast! Luffy!" Usopp was crouching too, although he'd already started in a half-crouch thanks to fear. But he was holding his tray in his hands now, thinking the plastic chairs would be too heavy and wear him out if he had to swing one too long against any nurses or orderlies who tried to stop their escape. Despite his protests, and as he'd known all along, he was with Luffy.