If atheism isn't a religion, then it's certainly a philosophy. Anyone who doesn't believe in religion probably has reasons for it, and to form a proper ethnoreligious group they'd need a shared reason/philosophy behind it. In the past few centuries in the Western world, it would be based on rationalism and likely adherence to ideas found in Revolutionary France, the USSR, and Mexico under Plutarco Elías Calles. Odds are to form a true ethnoreligious group, the state which produced them (i.e. the USSR, Revolutionary France, etc.) needs to have fallen, but they still identify with that state rather than its successors.