Attendees sign an NDA and cannot divulge what technologies and features which are displayed at the conference. If you hear rumours about particular technologies, someone is being unscrupulous.
I certainly get a very strong impression that many members of WRI are working very long hours up to and including the conference. I doubt there is time to craft a careful announcement about a diverse set of new things. I would guess that there is a great deal of quality assurance and consistency checks that need to occur before the technologies are released or promised.
I am in no position to guess when WRI would make an announcement, but I can appreciate why they would choose to be prudent about the timing of announcements.