Let's look closer at what the Apostles are saying there. "Not I am (what)"? What is the implied thing that they are saying they are (or are not)? They are not... the betrayer. We use the same phraseology today when someone asks, "Who is over 18?", and someone replies, "I am (over 18)".
But that is not what Jesus is saying. He says, "Before Abraham was, I AM!" I am... what? because there is no qualification to "I am", it simply means "I exist". There is no other person/entity that can say, "I exist" without qualification or explanation. Humans exist because of something (God). God simply exists. Further, even the tense of the verb does not match. Abraham "was"... so Jesus should have been "was". But Jesus says "am". He is saying that He, Jesus, existed before Abraham and continues to exist even now.
Jesus is clearly quoting Himself in what He spoke to Moses from the bush in Exo 3:14. He is pointing back to both Abraham and Moses, and bringing to the minds of His audience both of these patriarchs. He is making the case that He was the one speaking from the bush, and that even before that, He existed before the first patriarch, Abraham.
As for "I am" being different from "I be", that is a very, very weak argument. "Be" is one of the conjugative forms of the verb that "to be" (am, is, was, were, be, being, been). And God is not saying to Moses, "I will be", nor is He saying, "I have been". He is quite clearly saying, "I exist because I exist." And this is the same that Jesus is saying to the Pharisees, "Before Abraham was, I exist."