Flash Sideways Question

Just finished the series for a second time. Loved the ending, contrary to what everyone says, but I'm a bit confused as to what the flash sideways is.

At the end of Season 5, when the stop "the incident" from happening, I had assumed the flash sideways was a second timeline created to resolve the time loop. We see the island underwater, as though the bomb HAD gone off, and the resulting flash sideways storylines were what would have happened if the island didn't exist.

That being said, if this reading is true, why do we see Dr. Chang looking as young as ever in these flashes. At that point he should be at least 90? Also, Jack's dad calls it a place "they all created", so is this "timeline" a branching timeline at all or just a purgatory? And if so, what was the point of the end of S5, and what did they actually accomplish through that?