I agree to a point. I don't think everyone can get 18 inches, but I think people can get fairly large arms without direct bicep work.
I got to around 17 inches with never doing any direct bicep work and only close grip press and board presses for triceps. Once I added more direct arm work, I gained 1/2 inch in about a month.
But, I see no reason not to train your arms directly, especially if you train with your physique in mind. I didn't for a long time. Latley my mindset has shifted more and more towards bodybuilding (though I don't like to call it that). Just do some arm work if you want big arms. You will eventually reach a point where they won't grow from indirect work.