Without getting into exactly what security by obscurity means, I think what people try to warn against when warning against security by obscurity is advice that

Without getting into exactly what security by obscurity means, I think what people try to warn against when warning against security by obscurity is advice that distracts from solutions that do not depend on obscurity. For example the advice to move your SSH port distracts from the advice to use a private key and disable passwords. But in the real world, there's no mechanism as secure as public key cryptography, so it makes sense to keep the mechanism secret, as added security. The author's examples of dead drops or frequency hopping offer nothing close to perfect security, and it would be a real improvement to keep them secret.