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.