No. Atheism is not a belief. It is, however, a position of theism (not god). Theists claim there is a God. Atheists don't believe you. I don't claim there is no God. I simply see no reason to believe because theists haven't met their burden of proof.
Would you say that the belief that something does not exist is a belief or not a belief? I understand the reasons why so many of us atheists would like to claim that atheism is not a belief--there is no good reason to believe that any gods exist. There are also a lot of atheists who want to claim that the word "god" is devoid of meaning (igtheism). As a linguist, I reject igtheism, because it depends on an implicit theory of word meanings that is incorrect.
The OP gave us two arguments:
Premise 1: Having a God Concept makes any belief system a religion - if a belief system features a belief about God then it is a religious belief system
Premise 2: Atheists have a God Concept. They have a position on God, an opinion on God that qualifies as a position and an opinion on God, even though Atheists either see no valid reason to believe in God or explicitly reject such a belief. The point is, they still have God-beliefs
I would maintain that the first premise is simply false. Having a concept of what gods are is not the same as being religious,, nor is having an opinion about religion the same thing as being religious, nor is it even necessary to believe in gods in order to be considered a religious person. The flaw in Premise 1 is that it gets the concept of religion wrong, not the concept of gods.
I have no problem with premise 2. To me, that's the same thing as saying that people who believe that there are no leprechauns have a concept of what it is they are denying belief in. I would say that atheists do assign meaning to the word "god" and believe that gods, like leprechauns, are imaginary beings. They don't exist.