I have no source other than my own imagination. That's just the first thing I think of when I read that line (after having already played through the game of course). I think it's because I'm used to cryptic lines in games that don't ever seem to mean anything (FFVI "behind the stairwell" line, or TLOZ stuff the old man says about doors and stuff). I seriously doubt that Tifa is really referring to the Kalm Traveller, but he does live upstairs (in Kalm, though, not Midgar), he gives you some stuff which could be considered weapons, he's part of some secret sideplot, etc. Of course, I think they just added the WEAPON sidequests in the American version, so it would seem very unlikely that Tifa would even refer to that quest in story dialogue. But wouldn't it be cooler if she did?