Two poor examples of an axiom are not "on the same level" as axioms.
>Either of those axioms requires an assumption, and once you make such an assumption, everything else is filtered through that lens.
What assumption does atheism make? None.
From the cosmologists' point of view, it is not clear that the cause of our observable universe need be available to our study within that observable universe. There is no assumption being made; it is simply an unanswered question, about which we are unsure we will ever have a useful answer.