I don’t know a whole lot about Piranhas… but I know enough to know their reputation is grossly exaggerated.
I’ve never known anyone who kept Piranhas in a healthy aquarium and on a healthy diet that had ruthlessly aggressive fish. That is not to say Cichlids and Piranhas are good tank mates for one another…
Yes, I do believe that your Dempsey is learning the habits of your Piranhas. The theory of ‘dither fish’ is that when hiding Cichlids see the dithers swimming in open water not being harmed, it shows the Cichlids it is safe to come out… Well doesn’t it make sense the opposite lesson will also be learned?