Hear me out.
in "LA X" they start the episode off by showing us the Jughead explosion twice. The first time we see it, Juliet hits the bomb, we flash to white, and then we're in the plane with Jack in the flash sideways world. The plane doesn't crash, the island is underwater. This, it turns out, is a sort of purgatory world.
We then see Juliet hit Jughead again, we flash to white, and the characters have flashed ahead to 2007. Jack saves the island, the Ajira plane escapes, Jack dies... and goes to purgatory? Probably, since I think I subscribe to the idea that the plane Jack sees fly overhead as he's dying is not Ajira but Oceanic, and since we hear the flash sideways transition sound effect as it flies over, I take it to be his transition into the flash sideways world.
Soooo..... here's my theory. At the moment of the Jughead explosion there WERE two timelines created. One in which the bomb did explode and the island sank and everyone died and went to purgatory, a reality they created in which the plane did not crash. In the second timeline, Jughead did not explode, and the characters flashed ahead to 2007, and completed the Island storyline, during which Jack dies in the bamboo field.
The flash sideways WERE an alternate reality/timeline. AND they were purgatory. AND the Island stuff was real.