But Kate, Hugo, Ben and Sawyer never died. So why are they in that timeline?
Actually that's were you are wrong. The "purgatory" scenes we see are taking place after the death of all main characters. You could say it is sometime in the future, but I doubt that any notion of time exist in that place after all.
How do we know that the events there do not take place right after the death of the smoke monster?
Because the surviving characters talk about their time on the island long after this, where Hurley became Guardian of the island for an extended time, with Ben being his "number two":
Hurley continued to protect the Island for an unknown period of time, with Ben acting as his adviser. It is unknown how long his tenure as protector of the Island was. In the flash sideways Hurley would later comment that Ben was a good "number two." Ben stated that Hurley also did a great job as the Island's "number one."
(from Lost Wikia)