This was a mistake, IMO. It was irrelevant, you knew exactly what Fellow was talking about. There was no need to bring this up at all. You should have said something like "I do not believe that god does exist, which is different from believing that god doesn't exist."
I don't understand your objection here at all. Which god should we assume an agnostic is talking about when he asks "Do you believe in god?"
Seriously? What culture are you a part of? Is there no default religion where you are?
Yes, seriously. I'm surrounded by, and part of, several different cultures regarding god. If one of my religious family members asked me the question, they would mean the god that created the world, and led the Jews through a bunch of wars and insists on what I should eat and who I should marry, but absolutely did not come to earth in the form of a man. If it was one of my Christian friends who I debate with, they would mean a god who did a bunch of slightly interesting things, but crowned his mission by dying for my sins. They also may or may not believe that he performs a miracle every time they eat a wafer and drink wine in church, whereby they eat his actual bodily constituents.
These people are giving a (somewhat) coherent implied answer to the question "which god?", and I can go ahead and say that I outright do not believe in what they are claiming. The question from an agnostic, though, is incoherent, unless I know that they're a Christian agnostic or Buddhist agnostic or whatever. The broader culture doesn't help in a one on one conversation.