I mean exactly what I said, the NSA cannot do anything with SIGINT except collect it. This is not “due” to anything, it is an explicit constraint.
If one state’s borders were controlled by another’s, its operations reliant on another’s, and its leadership largely coming from or connected to the other’s, we would not consider that state sovereign. In the same way, my point is that the NSA cannot be considered independent of the CIA.
While other agencies can use SIGINT, all of it goes to the CIA by default and access to the NSA’s network is still gated by the CIA. The converse is not true - the NSA does not oversee most broader intelligence networks (shockingly neither does CYBERCOM), and the CIA also operates siloed networks and operations.
The military example you gave is a great way of thinking about it. The Army and Navy gather resources and maintain readiness, but operations and control of resources is afforded to component commands. All of these are technically separate and roll up directly to JCS/JTFs or DepSecDef. Similarly each intelligence agency is technically separate and oversees gathering/maintaining intelligence under its mission, but is still practically under the CIA’s influence.