barbo -- yea one of the key things that Christian tells Jack in that whole scene is "the concept of time doesn't really exist here". Like since their real world, practical deaths span over such a massive length of time, probably the easiest way to think of it is, whenever someone died earlier on in the whole saga (like Charlie, for instance, all the way back the end of season 3), imagine that like their spiritual consciousness instantly became frozen, alllllllllllll the way til whenever it is that the last one of the group had their death occur (like plenty of fans think that Hurley lived on for a couple hundred years at least as runner of the island, and mentoring Walt to eventually take over). But so that they basically all "woke up" in that sideways world at the same time so they could start interacting with each other, no matter when their literal death was. And then whenever one of them would experience a moment connected to the love of one or all of the other members of the group, they'd start getting flashes of their real island life, and remember who they were.
actually which reminds me, if you literally just did finish off the finale a little bit ago, you probably haven't seen this yet. Think of it as a post-credits scene, but it's like 12 minutes long. It's a little short film they produced that was included on the season 6 DVD set, and it takes place after the point of the end of the finale, where we see the Ajira plan taking off from the island with Sawyer and Kate and all them, and Jack dying still on the island.
https://www.youtube.com/watch?v=kH0P-CA7ntE