SmartThings originally said they expected to be HomeKit compatible back in January, before manufacturers realized how complex HomeKit integration would be. since then SmartThings, along with many other manufacturers, have backed off promising HomeKit integration.
If you’re reading articles and interviews, ignore anything before March 2015.
Current ST announcements are that they will consider future HK integration, but it won’t be part of Hub 2. Ever. (There’s a required hardware piece hub 2 doesn’t have.) Maybe Hub 3 eventually, but no telling one way or the other.