The logic here doesn't hold with respect to monothestic religions such as Judaism or Christianity.
The Ten Commandments, which are common to these two religions, have two "tables," the first of which (commandments 1 - 5) defines the relations between God and man and the second of which defines the relations between men (6 - 10). The very first commandment is "thou shalt have no other gods before me. The next proscription is against idolatry, which basically amounts to a worship of things that are seen, or the creation itself, which in KJV language is as follows: "Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth." In other words, belief in God is everything while belief (in the sense of worship) in anything else is all idolatry.
Thus, the logic of a monothestic religion of this type is that there is one true God and every other purported deity is a mere figment of man's imagination, hence an idol (the very word "idol" derives from a Greek root from which the Greek word "to see" comes from - even the word "idea" comes from a Greek word originally meaning "to see with the mind") and hence not real.
The logic of this piece holds if we proceed from an atheistic baseline, which takes the opposite view that only that which is material is real and all purported deities are only figments of man's imagination. If you assume that premise, then it is true that my belief in one deity and rejection of the 529 that others have believed in is not materially different from your rejection of all 530 of them (or whatever the number).
But this logic falls apart if one assumes there is one true deity (infinite creator and providential overseer of a creation from which the deity is above and apart) and all the rest of the purported deities are simply products of the mind and hence not real or true. In that case, the person who believes in the true deity can consistently say that he as a theist believes in the true one while rejecting all the false ones while the person who denies all of them includes in that denial the one true deity and hence is an atheist.
I know this is not the forum to get into religious debates but, if my summary of the logic is wrong, I invite others to explain why. Note that I am not assuming here that any religious point of view is true. I am merely taking that view on its own logic and explaining why it makes perfect sense for someone with that belief to say that he is a theist while one who is not a believer is an atheist. If the premise of one true God is true, it is utterly irrelevant to say that man has concocted a billion fake ones such that we are all atheists in denying them.
In other words, from the theistic perspective, the question is whether there is indeed a true deity of whatever type. If so, that is pretty important. If not, then obviously it is not. (Disclaimer: I am a believer and, indeed, a deacon in my church - but I am not making my points from a religious perspective, merely a logical one).
Edit (concerning the several replies): I invited you to engage the logic of my argument and you certainly did! My post is probably beside the point if all the original piece did was depict a way of steering away from a religious discussion that begins "How can you not believe in God?" or if it was merely a technique for getting a believer to understand the viewpoint of one who does not believe in a particular deity or in any at all. I don't think I was trying to exalt the theistic point of view or to denigrate those who don't accept it - merely trying to say what follows from accepting its premises. Got to rush now because I have to prepare to teach a Bible study tonight (really). Thanks for the fine comments - very stimulating.