This is ahead of the developer’s schedule, and a VP for BitTorrent confirmed that the released version was an internal development build, not suitable for public use – therefore they are considering that version a leaked version. Though they had planned to release a version themselves soon, some people weren't content to wait.
People are excited about the client coming to the Mac because it is a true port, not anything based upon existing Mac libraries. BitTorrent has cautioned people, and rightly so, that it is an unofficial alpha release and comes with no support whatsoever. Early reports look promising, though anyone who takes it upon themselves to use alpha software should already understand the repercussions.