You're badly misunderstanding the history. The Japanese didn't participate in Potsdam. The Allies wrote the declaration by themselves as an ultimatum and issued

You're badly misunderstanding the history. The Japanese didn't participate in Potsdam. The Allies wrote the declaration by themselves as an ultimatum and issued it to Japan, warning the Japanese of complete destruction if they didn't fully accept it. "Unconditional" doesn't mean "there are no terms." It means that the loser dictates no terms, while the victor dictates all the terms.