I think they mean the entrance to BC that isn't marked over on the Epcot side of the resort...not an actual entrance to the park Epcot. (If you do mean that, there is no such thing!) Many refer to the door as a "secret entrance" because its not marked as one. But yes, if you stay at YC you could enter BC through that door. It will take you down a long hallway to the BC lobby - where you would then have to exit to continue on to YC. You can't walk indoors completely from BC to YC.