He wishes he could say God definitely does not exist, but he can't.
Something along those lines, I think. He points out something that many theists have talked about, i.e., that the Big Bang could be the handiwork of God. But, as a 'negative atheist' (or, if you will, agnostic), he comes down on the side of seeking a physical explanation for the Big Bang.
Only in the same sense that he's "agnostic" about Elvis currently zipping around the galaxy in a flying saucer eating Krispy Kremes. Some claims, even if recognized as physical possibilities, are too unbelievable to be labelled with just "agnostic".
"Atheist" can refer both to someone who *lacks belief* in a god (soft atheist, negative atheist), AND to someone who *believes* there is no god (hard atheist, positive atheist).
Negative atheists probably usually have as much passion for atheism as they do for religion, and would prefer using the word "agnostic" as it seems less offensive to their religious friends.