That is simply a mistaken understanding. Keeping source code closed is obviously a legitimate form of security if the source code itself contains proprietary trade secrets (such as hardware details).
No reasonable person could disagree; the proposition is so self-evident it's virtually tautological.
Well, language and meanings evolve over time and you cannot control how they do it.
Also, how protecting trade secrets via closed source "creates a robust platform less susceptible to attacks than a CopyLeft-based system with GPL components."?
Does keeping the source code for the Death Star exhaust port controller firmware secret help create a robust planet destroyer less susceptible to attacks?
No reasonable person could disagree; the proposition is so self-evident it's virtually tautological.