The promise with Laboon was that the Rumbar Pirates would go all away the Grand Line and come back, which Yorki hasn't done, and Brook hasn't done yet. Otherwise Brook could just have met up with him at any time during the timeskip. So it being Yorki would be an added reward to Brook for achieving his dream- he doesn't just get to meet Laboon, but a friend he thought long dead.
As for why he's wearing Japanese style clothing, that'll be something to do with what he's been doing the past fifty years. Also a cowboy hat would be too obvious as to who he was, there wouldn't even be a conversation then. There's a decent amount of Japanese themed characters that as far as we know have nothing to do with Wano, and with how Wano's gone wouldn't really make sense for them to be from there- Sentomaru, Issho, Shiryu