That’s one of those rhetorical ‘have your cake and eat it too’ situations. I personally find it rhetorically more useful for classify atheism as a religion in discussions of policy, because that secures it protection under laws that secure freedom of religion. Arguments that Atheism is not a religion can be harmful when tossed about willy-nilly, and are pretty meaningless when the person making the statement doesn’t unpack the pet definition of ‘religion’ they are using to make that statement.
I’ve read works that posited that Christianity is not, strictly speaking, a religion, and in fact is at its core a philosophic system that directly contradicts how religious systems typically work (though in practice often perpetuates these systems rather than contradicts them). But the authors don’t just drop that rhetorical bomb and walk away. They spend a chapter (or a whole book) unpacking what exactly they mean by ‘religion’ and why the distinction they’re making is meaningful. Atheists, however, seem to mostly treat the distinction as self-evident and beg the question, which strikes me as pretty unreflective.
