I didn't submit so I don't know for sure, but it doesn't sound good for you. One reason festivals usually need to call filmmakers well in advance because many of their first choices will have already said yes to other festivals so they'll need to go down their list to the backup films until they complete their lineup.
If Rotterdam isn't as picky about world premiers, this might not be an issue though. But as a reference, you can pretty much guarantee that no filmmaker is going to refuse a screening at Sundance, which is also in January, and they still notified the selected filmmakers over Thanksgiving weekend. There's tons of paperwork and prep that needs to be done both by the festivals and filmmakers before the screening, plus it's just common courtesy to give plenty of notice.
I think the reason why they hold out for rejection notices is just in case a filmmaker backs out or becomes disqualified for whatever reason, so there's always a chance you could be the very next on their list. It's a cruel waiting game and I am in the middle of it right now as well for other festivals! Good luck!
