Philosophically, that's true. But I understand putting it in the religion column for the purpose of statistics. And 'none' and 'atheist' are two different possible answers. You can have no religion but believe very fervently in a god of your own conception.
Atheism is the lack of a belief in a god or God's.
Many atheists including myself adhere to the statement of simply being unconvinced that there is a God or having no credible evidence to indicate a God's existence.
Maybe the form we were filling out legitimately needed that information but typically outside of a very specific set of data. I don't see why any form would ever ask you what your religion or lack of religion might be.
Atheist is tangential to religion, you have religious theists or atheists as religion is about a system of philosophy and ethics while theism is about belief in gods.
Irreligious, 'none' or 'does not apply' might be better.
Atheist is not a statement about your religion at all. I'm also an atheist and my 'religion' is basically secular humanist. Yours could be none, Unitarian Universalist, Buddhist, secular humanist, or any others that are compatible.
The closest I have to religion is naturalist, which is to say based on the data I have, I can't find any evidence anywhere of supernatural effects, and while I haven't personally done the looking, the scientific consensus is that there's nothing, even when it comes to religious investigators actively looking for evidence of the stuff they expect to see.
To date, there are some pretty high bounties on evidence of various kinds of spooky shit that remain unclaimed.
Of course, I don't know this to be the case. The simulation hypothesis has not been ruled out, which would allow for effects we cannot detect. It doesn't stop me from worshiping my cat. (Cat-huffing. It's a thing.) And it raises questions whether we are the same person when we wake up after non-REM sleep, or a new iteration of the identity. (The transporter paradox). But so far, the most likely explanation is we're material girls living in a material world.
There are different categories. I consider myself agnostic atheist. That pretty much means I don't really know, but all this man made stuff is horse shit.
Atheism is a belief. You have a belief that there is no supernatural being or power. A lack of belief in anything is agnosticism, not atheism.
If you define religion as a belief (I would) then there you go. If it's a set of practices and dogmas, then technically Christianity isn't a religion and it's denominations are as they practice things differently and some have added dogmas or different interpretations, even though they believe in the same God and have the same fundamentals.
I'm fine with it on like a drop down or a form. No need to make things needlessly complicated. It's basically 'none' except it's clear. 'none' could be misleading, atheist is exact.
It's all a bit vague, but one could say "Atheist is a religion and Agnostic is none, because Atheists believe there is no god or afterlife" but often Agnostic is considered a type of Atheist.