I can see their point. One can view obscurity as "not common knowledge" or "information that can be found if you look hard enough."

A secret key technically falls in the later. You have it somewhere. If someone looks hard enough, they can find it. It may take ridiculous effort to the point of being impractical, but that is still obscurity.

I recognize that when people speak of "security through obscurity" they are referring to esoteric knowledge that is difficult but not impractical to find.

Ruan writes in his book in chapter 4 "The Engine: Safeguarding Itself before Safeguarding Others":

"In addition, there is a basic guideline for realizing security: Never rely on security through obscurity. When designing security hardening features for the engine, it is always assumed that all firmware source code and internal architecture documentation may be obtained by attackers. The engine’s security design principle is to harden the product by applying proven cryptography and security primitives, rather than rely on hiding secrets in the code or documents."

So they cast their secrets in silicon I guess, which would be incomparably harder to recover.