Usopp is oblivious to who the WSS happens to be even though he's a Warlord
On the other hand he knows who Shanks is, 'ofcourse' he knows who Shanks is
Look at the reactions of these guys to Rayleigh, a washed up legend who's been inactive for decades
Or Dragon
Or Whitebeard
Just passing by and droppin some canon truth bombs while I'm at it