I never thought it was suppose to be a bluff. I never understood people who watched the episode that thought it was a bluff. I think it was more wishful thinking because they didn't want their heroes to be making those types of decisions.
I actually never thought it was a bluff but this thread got me thinking and I allowed for the possibility of a bluff. In fact, the more I think about it, the more a bluff makes sense. A captain has to have 2 other officers agree to set the auto destruct on a Starship but a simple order from him is enough to exterminate an entire planet? I don't think so.
If such an order actually exists, I would envision it coming from Starfleet with multiple methods of confirmation and cross checking and even then, it would require 2 other command officers to agree to it.