"Sure. If you want to argue that there is a single belief in atheism, go for it. That still doesn't make it a 'set of beliefs' (presumably more than one belief), nor allow atheism to fit the various other requirements that makes a 'set of beliefs' a religion."
No, but it definitely occupies a particular, unique space in the world of religious thought that makes it qualify for discussion in something like this, even if reducing atheistic thought to a single axiomatic position (in this case, an active rejection of the thesis that a deity/deities exist) is a gross oversimplification.
This tangent is a bit dense because the original concept of the thread is dodgy, but if we're going to "rank" the world's religions, we would obviously have to set some kind of threshold of participation to determine a cutoff point (below which a particular belief system wouldn't have a sufficiently relevantly high number of participants to be worth the time spent discussing it). There's no doubt that the broad umbrella of "disbelief in existence of god/gods" has enough participants to merit inclusion in such a discussion, nor that if we broke down this broad umbrella into too many constituent parts, we would have too many such parts to consider and probably not enough followers in most of them to be worth considering.
So while it's obviously a gross simplification and inaccurate, a reduction to simple "atheism" is probably at least somewhat fair to make sure that the important space of "disbelief in a deity/deities" is covered in the discussion.
At that point we're just debating degrees, and I don't think I care enough to do it beyond this, but I think acknowledging and "ranking" (as it were) the substantial realm of disbelief in deities would be more informative than not, despite the complications that come from such an oversimplification.