I agree (overall) that theists tend to be "gnostic" (by which of course is meant that they think they know that a god exists; usually they argue that their personal -religious/other- experiences are proof, and so on). But the categories atheist and theist are not really antipodes of each other, due to the center not being an absolute center either. An atheist can think he knows that god does not exist (which is false), or think that god does not exist, due to arguments X. From there the atheist can be open to some degrees to examining if a god might exist. But he is still far away from having the position that he thinks there can be no evidence either way, and that there is no evidence which can set a breakthrough in a search for the existence (or lack of it) of a god.
Most atheists reason that they do not see anything that would make their stance less tenable, and so maintain a stance which already is one directed to a specific view (namely that there likely is no god anyway). An agnostic, in theory, has no set view on the issue, much like one might not have a set view on whether a billion star-systems away there is a singular or binary star in the center. It is just that religion has made the idea of a god seem to be about humans on a planet, so some humans expect that if a god exists then that god would have had to know/care about us all the same.