I think the flash sideaways timeline is Jack's creation. By the end of the series, a handful of the people who survive the events of Season 6 will escape the island by means of the sub or the Ajira plane while Jack will stay behind and take Jacob's place as protector of the island and jailer of Flocke. While doing this job, Jack takes a moment to think about what life would have been like if the stunt with the bomb in the Incident had actually worked and used his Jacob-like powers to explore that alternate reality. He even sends Juliet a vision of that reality to give her some peace before she died. Desmond bumsp into the reality somehow. After Jack's done with his exploration, we'll have the scene of Jack and Flocke sitting on the beach like Jacob and the MIB were doing in the Incident and we see from a distance another group of people heading towards the island. Flocke tells Jack how much he wants to kill him and Jack says he knows.
A new cycle is born.
A new cycle is born.