Note: the secret service will STILL have guns. It is merely the attendees who will be disarmed. There will be more than a sign enforcing it too. People will be checked as they enter.
You are forgetting that the secret service will have guns. There is still armed protection/enforcement. If you "catch" someone trying to bring a gun in, and there is NO armed enforcement, realistically m, who is goimg to stop them?
They're just complying with the law. He'd still be safe in a crowd of however many NRA members - unless some idiot non-NRA wanted to false flag an attack to pin on the NRA - which would likely be a probability comfortably above 1.00 .