I think that might depend on which version / definition of Atheism one uses.
For example, there are different forms of Christianity, as defined by different people.
The same evidently is true of Atheism.
A person that does not know God, is immoral, whether that person is ignorant of that fact, or not.
No person is born morally upright. From the time one leaves the womb, they have no moral values... well... even before leaving the womb.
I'll leave the question of how one becomes morally upright, for if you ask, but another question arises... how does one gain a conscience that tells them something is right, or wrong. Where does that come from?
The apostle Paul said...
[Persons who don't know God, do] demonstrate the matter of the law to be written in their hearts, while their conscience is bearing witness with them, and by their own thoughts they are being accused or even excused.
In other words, people have a conscience, which allows them to examine themselves, and render judgment about their own behavior.
Yet. we hear the expressions... 'that person has no conscience.' Or, 'that person's conscience is dead.'
Everyone's conscience does not dictate the same "laws". So what does that tell us? The conscience is trained. Or, it can be corrupted or dulled.
So, we are led to another question. How can a conscience be trained to be accurate?
You see, your question has opened a can... or maybe cans of worms.
I believe there is only one question, and answer, that removes those cans for good.
The question is actually quite ancient. The answer is clear to millions.
Only one person can conclusively demonstrate the answer to the question to be definitive, though, so it would be to open another can of worms to discuss that here... and we don't want to do that. Or do we...