Cheese Pizza actually started out as a way of making fun of people requesting "cp" on places like 4 chan. Because 4chan had a cp problem.

There's a site called knowourmeme that does like internet history for memes.

More than 5 years before pizzagate, they started a page for the meme Mods are Asleep https://knowyourmeme.com/memes/mods-are-asleep

"Mods Are Asleep" is an expression used to indicate that a website's moderators are currently inactive and unable to enforce the site's rules of conduct. The phrase is often followed by a call to action for other users to post material that is typically not allowed.

And people would say "post cp" https://i.kym-cdn.com/entries/icons/original/000/003/202/imagesCABYJCBO.jpg

Two years before Pizzagate, the site explained people began trolling people looking for actual CP by posting things like chocolate pudding, copier paper, Captain Picard and yes, cheese pizza.

About Post CP is an acronym in which CP can have a variety of meanings. Initially used as an acronym for 4chan posters to indirectly mention or request a child pornography thread, in later years the acronym became commonly associated with the posting of other subjects with the same first letters (C & P, most notably "Cheese Pizza") to parody prohibited request threads or to troll those who hope to find child pornography.

Origin 4chan's anonymous posting style has long been a way for users to request and post socially unacceptable content and taboos without facing many risks. Although 4chan houses content that other areas of the internet wouldn't post in public, the site still maintains a set of rules which also forbid the posting of "anything that violates local or United States law". Regardless of this, users commonly try to post such content with the hope the site moderators won't spot it, such as in "mods are asleep" threads.

It's hard to understand this whole thing without this background and the context in which this started.