This is interesting and I totally missed it earlier. This would indicate that they now don't have enough people booked in this frame of time and will need to open reservations for this period in order to meet their desired capacities.I genuinely don't know when previews will happen. My personal speculation is that CMs will have previews either right before the 4th or right after with APs and DVC right after them, but again, that's just a guess. I don't know.
I will say more confidently that we should hear more concrete information on resorts and reservations by hopefully the end of next week (could be earlier). The plan last I heard was for any remaining DVC/Deluxe Villa cash rooms to be available for the general public to book during the initial reopening period (6/22-7/4ish), but that could change.
ETA: They are really cutting that down to the wire too, if true, because it's already less than 2 weeks before DVC resorts open and they so far haven't announced general public new reservations bookable for this time.