Atheism is irrational.

[Asmodee]

When you group people together based on a label that label has to describe THE ENTIRE GROUP.  You cannot assign meaning to that label based on what a few, some, half or even most of them are like because IT MUST DESCRIBE ALL of them.

It depends of whether the proposition in question is a definition, broad generalization, normative description, or a statement about the average. A broad generalization can be accurate even if it is not true in every instance. You must also take into account whether the noun is used in a way that refers to a category or in a way that refers to a type.

You're arguing semantics, and not very well.  "Atheist" is not a broad generalization, normative description or statement about the average when you label someone an "atheist".  That's like saying that when you say, "black people" you aren't necessarily talking about people of color, or when you say "Muslim" you aren't necessarily talking about people who are Muslims.  The discussion was about what it means to be "atheist", what the definition of "atheist" is.  As a definition, that definition must be such that it includes every person defined as an "atheist".  If you change the definition you change what it means to be an atheist, you change who is and who is not an "atheist".