As has been pointed out, atheism doesn't get to be so special that it can mean two different things.
The confusion about it's definitions can be traced to this declaring that it can cover two differing positions in order to force us to accept that there can only be theists or atheists re the question of GOD.
I consider that to be a deliberate lie, in regard to those who purposefully tell it.
"Atheism is the condition of not believing that a God or deity exists." is untrue.
Non-theism is the condition of not believing that a God or deity exists
Theism is the condition of believing that a God or deity exists.
Atheism is the condition of absolute ignorance re the question of GOD. Once the question becomes understood and a deliberate choice to believe one way or the other [GOD exists or does not exist] then one shifts from being an atheist to being a non-theist or a theist, or remains undecided re the understood knowledge re the question.
No one born, remains totally ignorant [as the default position]. Therefore no one remains an atheist.
The idea of using the same to describe another position ["I am an atheist AND a non-theist"] is where the source of the confusion lies.
As such, atheism is ill-defined and I am confident that I am neither atheist
nor theist and that being
neither is a legitimate position to hold regarding the question of GOD.