What I think he means is that they can’t change where they have been or how they got to where they are. They can think that they are trying to change something, as Daniel does with young Charlotte, but it ultimately it won’t matter. Everything they do, they have done, and has already been accounted for.
Imagine the main original timeline, the one we are seeing and the one that the real world operates on, as straight line. At the point that the flashes begin draw a loop in that line; this is the Losties’ timeline. Nothing that happened before, or on, that loop can be changed. However, anything beyond the point that that loop starts, and ultimately, the end of the line can be shifted/changed.
The real world continues on its steady timeline, but at the flashes the Losties have gone off on a detour, the loop.
Desmond was “special” only because he was in the real world/post-loop time.
From our perspective the Losties are in the past, but, especially now since the flashes have stopped, no matter *when *they are, they are technically in the present. From a first person perspective, they are still on their own original timeline. The futures of these original timelines have not yet been revealed, but nothing can effect what has happened on them already. What can be effected(or changed, but then you get into the whole ‘free will v fate’ thing) is everything that happens after they started the jumps; i.e. this and last season’s off island stuff and the 316ers time.
The outcome of this coming war could be(or has been) affected by the Losties’ actions, but the actual effects can not really be seen until our(the audiences’) and the 316ers’ present and future; anything beyond the point that the loop started.
Their dopplegangers, the other ‘thems’ created during some of the flashes, might as well be shadows or trails. Or ghosts. They will follow the same paths, say the same things, and make the same choices that we have already seen them make. I suppose you could call them reruns.
Sort of like in Primer.
Does that make sense?