So if I hear you right, you're saying the hardcore atheists are saying, "I know exactly what god you believe in and I categorically do NOT believe in that same god!"<quoted text>
True, but the idea is that the harder core of the atheists are first presupposing something, and then disbelieving it. The statement that I dont believe in the same god you dont believe in, is, in part, trying to highlight that irony.
In that case I would think your response might be, "You have no idea what god I believe in." As mine was to you.(not to tell you your business, or take the fun out of it..) Regardless.
I doubt most atheists take that position, as you say the hardcore. I think most atheists and most theists take that middle of the road "I don't know position" but choose to believe (or act as though they do) or not as they see fit.