Irrelevant: you were after an atheistic society, not merely one that is viewed by a theistic society as getting their morals from God.
They could (and undoubtedly do) say the same about all atheists - that they still get their morality from God.
But the question was whether there were atheist cultures for comparison.
Do you consider animals to be theistic?
The crux is that whatever question we ask about this, it is already going to imply that we have taken one side or the other, the theist or the atheist one, as otherwise, we couldn't ask that question.
By asking "Is there a completely atheistic society?" and seek a Yes or No answer to this, already involves us deciding, definiteviely, whether we are going to take the atheist or the theist outlook, and if the theist one, which one in particular, as different theisms differ on this.
So rather than asking "Do you consider animals to be theistic?" or "Is there a completely atheistic society?", we'd first need to clarify which side we're going to take.
We don't have any true test culture; unless we posit that animals do not get their morality from God, aren't created by God, or aren't embodied souls. I don't see how we could possibly posit any of this. Animals would be such a test culture if we took the mainstream Christian stance or the mainstream TOE stance, but not if we took an Eastern one.
Anyway, the whole issue of religion being the source of morality and that people without religion would be brutes seems like a typically mainstream Christian thing. I don't know about Islam and Judaism; I only have a limited knowledge of those, but from what I do know, I don't recall them proposing that argument.
Then, going East, they make it more complex by introducing the gradation and variation of religion, and how the term has several layers of meaning.
My point was that one should not find it surprising (as you seemed to be) that people raised as practicing Catholics do not know their Catechism word for word.
?
At least from my background, the notion of a "proper formation of conscience" always seemed a very pertinent one, one that one practising Catholicism would be as familiar with as with Jesus being the Son of God. It seems of such vital importance!
It's also what I thought of reading the OP, given that Catholicism addresses this very explicitly.