Two points.

First, I've also heard antitheism defined as the position that religious belief is harmful (think Hitchens book). So that one could without logical contradiction be an antitheist and a theist.

Second, my understanding of the current experimental data is that humans have hardwired religious tendencies that would make an Untheistic society very unlikely to appear. Obviously it works for a thought experiment but but saying "Babies are natural atheists/untheists" is problematic because while that might be true strictly speaking babies are likely natural animists and possibly immortalists.