People who claim atheism is a religion are:

[beam me up scottie]

8. Atheism is defined as the absence or rejection of belief in gods.

It doesn't attempt to explain anything.

From American Atheists:

WHAT IS ATHEISM?

No one asks this question enough.

The reason no one asks this question a lot is because most people have preconceived ideas and notions about what an Atheist is and is not. Where these preconceived ideas come from varies, but they tend to evolve from theistic influences or other sources.

Atheism is usually defined incorrectly as a belief system. Atheism is not a disbelief in gods or a denial of gods; it is a lack of belief in gods. Older dictionaries define atheism as "a belief that there is no God." Some dictionaries even go so far as to define Atheism as "wickedness," "sinfulness," and other derogatory adjectives. Clearly, theistic influence taints dictionaries. People cannot trust these dictionaries to define atheism. The fact that dictionaries define Atheism as "there is no God" betrays the (mono)theistic influence. Without the (mono)theistic influence, the definition would at least read "there are no gods."

Why should atheists allow theists to define who atheists are? Do other minorities allow the majority to define their character, views, and opinions? No, they do not. So why does everyone expect atheists to lie down and accept the definition placed upon them by the world’s theists? Atheists will define themselves.

Atheism is not a belief system nor is it a religion. While there are some religions that are atheistic (certain sects of Buddhism, for example), that does not mean that atheism is a religion. Two commonly used retorts to the nonsense that atheism is a religion are: 1) If atheism is a religion then bald is a hair color, and 2) If atheism is a religion then health is a disease. A new one introduced in 2012 by Bill Maher is, "If atheism is a religion, then abstinence is a sexual position."

The only common thread that ties all atheists together is a lack of belief in gods and supernatural beings. Some of the best debates we have ever had have been with fellow atheists. This is because atheists do not have a common belief system, sacred scripture or atheist Pope. This means atheists often disagree on many issues and ideas. Atheists come in a variety of shapes, colors, beliefs, convictions, and backgrounds. We are as unique as our fingerprints.

https://atheists.org/activism/resources/what-is-atheism