I am probably a little dumb, i read the article but dont understand what happened. can some HNer kindly explain?

I guess that if you GET https://somedomain.com/boobs.jpg you get a 404 (not found) from a web server hosted outside of Iran but if the server for the domain is hosted in Iran, you get a 403 (forbidden) because the request is intercepted by a firewall that detect the word "boobs" and reject it with a 403 without forwarding it to the webserver that would usually return the 404.