Improbable situation. We have 3 options, BA is Light Yagami, BA is not Light Yagami, if he is, then FT either made an excellent guess or a not so excellent guess.

1: FT knew a death note character might be floating around because he's one
2: FT has watched the show and so has Yuna and its a pleasant coincidence
3: FT knew BA was Light because their masoned/scummed/ etc
4: FT has watched Death Note, and pitched it out there because it seemed vaguely appropriate, we discussed it a lot and BA decided it was a decent claim to hide behind, he's vig'd before, some players found Light as vig plausible

FT's good, but go back to the original message, there's really nothing there to point to Light, and it strikes me as just a loose guess by a player who had limited exposure to anime of a popular non-good character. 2 and 4 seem more probable than 1 or 3 only because I can't see FT, if he knew as L, saying that, and I can't see FT, as mason or scum buddy, outing his ally so horribly blatantly then ripping him up as evil. So, that's leaves coincidence or conspiracy. I think Hi/BA decided our talk of vig option made it a safe role for someone who is viewed as having done well as a vig. I don't think he's Light, I'm not sure he's a scummer, but I think he's lying.