This means some people don't understand double and triple-negatives, nor care about using correct English.
I cannot say that I do not disagree with you.
I would interpret it as "There is no way I am not going to say anything to someone." Meaning: I will definitely say something to pretty much everyone. It's really bad, uneducated speach. Don't use it.
I hope you're not agreeing to disagree with the opposite of the negative of what I did not say.