Is the Atmos really a boiler, meaning it boils water and works under pressure?
No. The Atmos is a hot water boiler. It heats water to only 180◦ F., well below the boiling point, so no steam is produced. And depending how the boiler is installed, it works at no pressure (open system) or very low pressure (closed system). As a safety feature, each system has a relief valve that opens if the internal system pressure rises to 30 psi, which it wont do in normal operation. As a further precaution, each boiler is safety tested to an internal pressure 75 psi before it leaves the factory.
