Senjirou didn't wanna reveal not even to the scabbards. So the answer is simple: Denjirou tries to find the traitor!
Thats why I bet that we with Denjirou revealing himself infront of the others also got the traitor.
I hope Oda reveales the traitor in the next 3 chapters. As you said Nessos Denjiro must know about the existence of a traitor, but i doubt he knows the true identity of him. I could be wrong though, but if Denjiro doesnt know who is the traitor, then Orochi and Kaido's alliance have a very important card as its own play. If Denjiro knew about the identity of a traitor, even just the existence of him, why wait so long? Why he dont alert to the raid (in some way, not revealing himself)? Or maybe he did it and we dowsnt know.