This is why I love One Piece— this crew’s absolute need for and utter confidence in each other.
Sanji knows Usopp has lost all faith in his abilities as a fighter and his feeling of belonging with the crew. He’s even resorted to hiding behind the mask of an alter-ego for strength.
And yet, Sanji tells him that it’s okay. There are limits to what a person can do.
Sanji’s no good at precision-based distanced combat. His strength only extends as far as his legs (which granted, are pretty long, but still) and that isn’t enough to help Robin in this situation.
So Sanji, one of the Monster Trio, asks Usopp, not Sogeking, for help, because he is powerless in this scenario. They need Usopp and his long-range sniping abilities to reach Robin. He knows that deep down, Usopp hasn’t lost his abilities as a sniper, hasn’t lost his pride as a pirate. He just needs a little push in the right direction. Because he knows that deep down, Usopp is still the Brave Warrior of the Sea and he will never abandon his nakama.