I don't think that's true. I can find no reference to "cromulent" existing pre-Simpsons, and every source explicitly says it was coined by David Cohen for an episode.
From Wiktionary: "A humorous, intentionally morphologically opaque neologism coined by American television writer David X. Cohen for 'Lisa the Iconoclast', a 1996 episode of the animated sitcom The Simpsons."
From Merriam-Webster: "It is safe to say that The Simpsons has contributed a great deal to the English language. One famous example is cromulent, which was coined specifically for the 1996 episode 'Lisa the Iconoclast'."
From Wiktionary: "A humorous, intentionally morphologically opaque neologism coined by American television writer David X. Cohen for 'Lisa the Iconoclast', a 1996 episode of the animated sitcom The Simpsons."
From Merriam-Webster: "It is safe to say that The Simpsons has contributed a great deal to the English language. One famous example is cromulent, which was coined specifically for the 1996 episode 'Lisa the Iconoclast'."