It's not correct to say we are all born atheist.
The tabula rasa does not default to a positive or negative view about the existence of God/gods.
We aren't born 'believing' that there's no God.
Nor are we born believing that there
are gods. So that puts babies in the middle, between the two kinds of believers.
What do we call those people in the middle? Some call them agnostics. That's awkward and confusing, but it isn't indefensible. Common usage and dictionaries support that usage, so it isn't exactly wrong.
But there's also another popular system of nomenclature. According to this usage, all non-theists are atheists. The atheists who believe gods don't exist are called strong atheists, and the atheists in the middle, the ones who don't believe either way, are called weak atheists.
This usage is also supported by common usage and dictionaries. So it can't be called wrong either. And it is overwhelmingly popular among those who identify as atheists. So, when you're talking to atheists, there's a presumption that "all non-theists" is what they mean by "atheists."
Further, it wouldn't make sense to claim that babies are born believing that gods don't exist. So, clearly, anyone claiming that babies are born atheist is using the all-non-theists definition of "atheists."
You wouldn't go to a website frequented by black people to claim something like, "You are not blacks. You are Browns." They get to decide how they want to think of themselves and what they want to be called.
Likewise, you wouldn't go to a male-to-female transexual website and insist, "You are not women; you are men." That would be the height of bad taste. They get to choose their own identities and labels. You may elect to think of them as men, but you wouldn't tell them that they should think of themselves as men.
So, unless this was an oops-I-didn't-understand-that moment, I think your post wasn't tactful. We atheists don't need a lecture on what our label means.
If you wanted to point out that you use the word differently than we do, so
you don't call babies atheists, that would be fine. But that isn't how your post struck me. It seems to me that you were correcting us, straightening us out, telling us how we
ought to describe ourselves.
That's not a move I recommend.