There is no such "Pun" in the Japanese version.
He just sings "Sore Nigero" He's just telling SOMEONE to Run. Considering Usopp wrote the song in a way to make Sogeking Look cool, he's most likely talking to the other guy.
Could he be talking to himself? yes, Easily
Is there something in the Japanese that says he HAS to be talking to himself? No
Is there some kind of clever pun in the Japanese that is incorrectly carried over if you don't make it look like he's talking to himself? No.I doubt Usopp, a man whose lies are made NOTORIOUSLY to make himself look cooler ("8000 Followers!" "Remember all those Baroque works people our crew took down? It was all done by ME!" "Five Tons!") would add something into one of his lies that makes himself look any less impressive.
It's actually somewhat out of Character for Usopp to do something that would make him come across as a cowardly within one of his own lies/stories, even if it's in Song form.
In Reality, he's a complete chicken who constantly gets "I can't get on that island disease".
In his Lies, he's the coolest dude ever who can hit any target and has 8000 followers.
Oh haha i must be reading into it too much :wassat: