> Could someone explain why this wouldn't fall under freedom of speech?
North Korea is under international sanctions and part of that means you can't aid them. Explaining how to launder money and evade sanctions to a general audience is probably fine. Explaining how to launder money and evade sanctions to North Korea is against the law.
You can almost certainly get away with "If I were North Korea, here's how I would launder money" as an academic article. Presenting it in Pyongyang seems like a pretty bright and clear "Aiding and abetting enemies of the US"
North Korea is under international sanctions and part of that means you can't aid them. Explaining how to launder money and evade sanctions to a general audience is probably fine. Explaining how to launder money and evade sanctions to North Korea is against the law.