Zack Snyder was too busy filming Justice League to actually be at the event, but he and the cast (except for Affleck who actually was on hand for a big announcement) shot a video message to the fans in attendance. And right there alongside everybody was Henry Cavill. Maybe it would have been a good idea to slide him over a couple of feet so he'd be out of frame, but no, there he was basically confirming that Superman will be in on the action.
It's always possible he's there just shooting flashbacks or something, but does anybody really believe that? Of course not. Justice League Part One opens November 17th 2017.