There are plenty of reasonable definitions of religion that would apply to atheism, but even that aside, it's a-theism, not a-religionism, it's a rejection of theism, and while many atheists do come at the topic from a generic anti-religion angle, there are many atheistic religions, like The Satanic Temple, who are doing fantastic legal work as a officially recognized religion to fight the current right wing legal nastiness like the abortion ban in Texas by arguing they have religious freedom to preform abortions.

So you're right in the strictest sense that the philosophy of atheism is not de-facto a religion, but next tmme I would recommend making an actual argument rather than incoherent swearing at someone with a mildly spicy take, you're making us all look bad.

Depends on context. In regards to a lot of Federal discrimination legislation like that in Canada, atheism is actively protected under the subheading of "religious belief".

In some Provincial discrimination legislation it is covered by the term "creed" rather than religion but if something goes to the level of the Federal Supreme Court then for purposes of protection against discrimination or for matters of hate speech atheism is considered for all purposes legally a religious belief.

Linguistically laws often term Atheism more broadly as a set of spiritual beliefs. Believing entirely in the absence of the supernatural is still describing what you believe about spiritual matters. Atheism is not a complete absence of belief. You still have things you believe about spirits - you believe they do not exist. If you truly had no spiritual beliefs you would have to be completely unaware that the concept of the belief of the spiritual exists at all including disbelief in the spiritual.

Fundamentally law that applies atheism as a religion employs this logic despite religion having connotation of an internal dogma - something absent in atheism.