I assume it was not admitted by the defendant (photographer). And the problem is that anyone can sign a contract and later claim that they had some oral side-deal. If such claims were admissible, no written contract could ever be enforced without a full "he-said-she-said" jury trial. In order to promote commerce, the courts have held that a written contract is enforceable even when one party claims their was an oral side-deal that was different. (There are a few exceptions to the rule, but not many.) This sometimes let unscrupulous people get away with things they shouldn't, but the alternative is worse.
My reading of the article was that the oral agreement was not in dispute. I could well be in error on that point.
If there is a dispute, you are absolutely 100% correct.