Gridiron is rarely used in the US as a substitute noun for football. The original article in this post was not from the United States. It was from the Daily Mail, a prominent newspaper in London, which is in England, which is in the United Kingdom, which is off the west coast of Europe.
In just about every country on Earth that isn't the US, "football" means something else. It means what we call soccer. So they need another name for American football to avoid confusion.
Now, can anyone tell me WHY the word "gridiron" refers to the game of American football?
Look at the yard lines of an American football field. Look at your barbecue grill. There ya go.