Either confidence or a bluff. He's not dumb, so he probably figured out they know about his big weakness. So now he's got nothing to lose. By admitting it they will be insecure or he really just wants them to have a real chance of killing him. That's just how he rolls.
I think Koro-sensei likes challenges, lol. He looks serious about destroying the earth, but also enjoying the time as a teacher. And he also wants to be stopped, so he probably reveals his secrets casually because of that. This will make the battle a bit more fair as students know of his weaknesses more, hoping to bring in something more interesting. Or so I guess. :P
He seems to be most overpowered in current arc. Though I'm pretty sure he already has some tricks to get out of it, to see students coming to his rescue would be cool. Looking forward to next chapter!
*stays away from spoilers, lol*