True, but do we know WHEN exactly the new castaways are? I mean, I'm assuming the present, but we don't know for sure yet, do we?

We know that it must be later than Seasons 3 and 4, because Caesar was going through what is apparently Ben's office on Hydra (so I've been told. Apparently there were cut scenes in S 3.), and it looked abandoned. Also, they have the canoes, which at some point will show up on the beach camp side of the island, where they will be found by the Leftbehinds - and judging from the camp's condition, this was probably somewhere between 2005 and 2008. I assume we will get a scene mirroring the one where the LBs run from the people in the other canoe at some point, and given that he is the one who has the shotgun, I suppose the person shooting at the LBs will be Caesar.

No idea why Sun and Sayid would have ended up in the future, other than it is plot convenient.