It's not a bluff for the simple reason Natsu doesn't bluff. He might easily overestimate his own strength, but he doesn't bluff. If he says he has a secret technique that can beat Zeref, it means he believes it can do that. Whether he would truly be capable of that, is a different matter. And till recently Natsu didn't know that Zeref was an immortal and he doesn't care about such details. He simply wants to beat the crap out of Zeref, which is still something very differently than killing him. Likewise it's not because you're immortal, you're unbeatable.
There's also absolutely no point in bluffing.