Yes, there was a lot of confusion going on there. There were many other functions happening at that time; where there was no LE protection. Add to the fact POTUS wasn't moved until after the shooter was in custody. Who knows how comms were handled that could have led to the confusion.
There were guests staying at the hotel that were using the same entrance as people attending the WHCD event.
Those people weren't being checked by SS or LE as they walked in to get to their hotel rooms.
This event should've been planned at a more secure location; especially since so many threats are received in today's world.