I agree that atheists deny the existence of any god, but there’s a burden of proof issue to consider. It’s up to theists to propose the type of deity they have in mind. Then skeptics can get to work on seeing if that deity makes any sense. There’s no need to deny what’s never even been conceived of.
As Richard Dawkins points out, there’s no need for an atheist to be absolutely certain about the nonexistence of any possible god, most of which, as you say, haven’t even been proposed. The denial of gods can be probabilistic. We can say, based on the poor track record of the major religions and cults, in terms of the incoherence and preposterousness of their myths and deities, that there’s probably no such thing as a god in the theistic sense of a perfect person who created the universe and whom we ought to worship.
I think you’re making a strawman out of atheism. You’re saying atheism is reckless because it takes an absolute stance on the unknowable. Atheism can be probabilistic rather than absolute, however. If we’re talking about what no human can imagine, we’re not talking about a god in the theistic sense since such a god has indeed been imagined and defined. There’s no work for the atheist when it comes to deciding on abstract, mystical forces, since those aren’t well-defined.