First of all, that sentence is not a permission for lie, it means you will hide your tactics from the enemy.
secondly, even if we accept it as a permission for lie, it's limited just to the battlefield and doesn't apply to peace or ceasefire, cause then it would mean breaking the oath. Qur'an mandates the Muslims to keep their oath even with enemies. in Syria, whenever the terrorists wanted to make a deal with Syrian government, all they wanted as a guarantee was a verbal promise from Iranian commanders.
thirdly, even during Iran-Iraq war, when Saddam was widely using chemical weapons, Iran didn't use chemical weapons. Couldn't Iran use it and then deny it ( as you suggest)? but we didn't, cause our leaders have forbidden the use of mass destruction weapons, it's a fatwa for Muslims, not enemies.