Just in case — he didn’t have the authority to launch the missiles. His job was to evaluate the situation and give the immediate report to the top of the chain

Just in case — he didn’t have the authority to launch the missiles. His job was to evaluate the situation and give the immediate report to the top of the chain of command, if the situation is critical, and if they decide to counterattack — relay their orders (and presumably some unlock codes) to missile operators. In other words, he was at his place exactly because the command authority didn’t trust the system enough to rely on automated reports, and needed a human in the loop to make the interpretation for them. He decided (correctly) not to relay anything to the higher ups. However, later he was not commended for that, because his job was to make the call and tell something like “I have an alert of five incoming Minuteman missiles — I am sure though it is a false alarm from the new system”, instead of unilaterally deciding to block the message. He was right not to escalate, but if he did, it would not have immediately ended the world.