If I find myself in a situation where, if I don't get X, I prefer to leave, then an ultimatum is an obvious course: "give me X or I will leave."
It can be to my benefit in negotiation for the other party to think that I am in such a situation, and so I may still say "give me X or I will leave."
If I do not intend to follow through, I am clearly bluffing.
If I intend to follow through, but absent this gambit I have preferred to stay even in the absence of X, I think we can still call it a bluff in a weaker sense - I am not misleading about my future actions, but I am misleading about the strength of my preferences.