The simple lack of belief in God is not Atheism.What the people pushing for the "belief" definition are actually contending is that there are few, probably no, such people - that "simply" lacking a belief in something that operates as a deity in one's belief system is rare, perhaps impossible.
Atheism is the belief that God does not exist.
The use of the term "atheism" to signify those who lack the belief in God is a Faux Pas proliferated throughout the internet. Such use for "atheism" is invalid.
An "ism" cannot signify somebody that lacks in a belief in something.
For example, everybody would probably lack the belief in a FSM. You cannot categorize them under an "ism".
Such use of the term "atheism" to signify a lack of belief in something is and has always been invalid.