"lack of belief" means that if they lack the belief for whatever reason...they are Atheists.
What "lack of belief" seems to mean is "a term that uses the word "belief" in a unique way to define the position of some atheists". Generally speaking, when people are asks if they believe in something and they have no idea what it is, they say I don't know. However, let's look at how this term is used in one of the only contexts it is (to describe the position of atheists):
"An atheist is someone who does not believe in any gods, that much is clear. Lack of belief in something will ordinarily have two causes: inattention and skepticism. That's why two main varieties of atheism are constantly promoted. It is crucial to grasp that "not believe that god exists" is different from "believing that god does not exist." Both positions are genuine kinds of atheism, and may conveniently labeled as "apatheism" and "skeptical" atheism."
Shook, J. R. (2011).
The God debates: A 21st century guide for atheists and believers (and everyone in between). Wiley
Notice that "does not believe in any gods" here is perfectly compatible with "lack of belief", as it must if the term is to be meaningful at all. The term "lack of belief" is used merely to distinguish between forms of atheism. I believe I have heard these two types referred to as strong vs. weak atheism, with weak atheism corresponding to apatheism described in the quote above.
Personally, I don't care for these as it is difficult to differentiate agnosticism from apatheism. Agnosticism means one does not know if any gods exist, and thus cannot be said to believe in any gods. To that extent they don't believe there is a god, for if they did they wouldn't be agnostic. However, this is still better than claiming that one can lack belief in god without believing no gods exist.
However I hope that no one is arguing against the idea that we are taught religion as a learned and indoctrinated event in our lives (especially early on).
No. But there are many (as far as I know, all of them religious) who believe we somehow "are" members of some religion or more commonly are at least somehow possess a connection to god from the beginning. Unfortunately, one cannot use critical argumentation against such beliefs.