"The weakness of "security through obscurity" is so well known as to be obvious. Yet major hardware manufacturers, citing the need to protect intellectual property, often require a non-disclosure agreement (NDA) before allowing access to technical documentation. "

I believe the actual reason for "security through obscurity" is that it's a delay tactic used against well-funded adversaries.

There's an inherent problem in security. A company, existing in the private sector, could never hope to overcome the infinite resources of a nation state. It's literally, mathematically, financially impossible.

A nation state could even apply a rule like, if they know a particular technology was developed by roughly 500 engineers at some company, a nation state could employ 5x the number of engineers used; simply as a rule. So in this case, they could employ 2500 security researchers to overcome some security problem.