Is this what atheism is, rationally speaking?

What you say is true for many, but not all atheists. One can be atheist and be committed to a life of total reason and empiricism. The ultimate question of meaning has no value as it's unprovable and indemonstrable. Thus, the question has no meaning and is not considered. This position is not existential or nihilist as both groups offer answers to this question -- rational empiricists do not. Now is rational empiricism delusional; it is another option.