I assume he just guessed. He didn't pick LM obviously. I don't think he went for Bonnie either, since she's gonna die anyway. So that leaves Kagura.
Ekko, like I said is double save - if Juliet lies about her role, she's WQ and if she's saying truth, he can't be WQ and I'm sure Fuji knows that. Idk about Juliet, all he said was that she's not Red. Could go with her in case he's reading our posts rn. Meeyori is basically confirmed at this point. Mitch is aslo confirmed. He said that he trusts me, so I doubt he picked me. Pot would be just really random person to choose.
tl;dr: I assume Fuji guessed that WQ is Kagura and guessed wrong. Or maybe he went for Juliet based on our posts, but I guess Kagura.
darling