This is basically it.
Imagine, for example, a case is about someone disclosing the location of nuclear arms to their spouse. Maybe even accidentally. If they are brought up on charges about this, national security doesn't want the fact that a case is about nuclear weapons to be public information. They further don't want it disclosed who was told about them (as the spouse wouldn't be subject to charges). That's basically where a secret interpretation and ruling would ultimately enter. The courts can't disclose what the ruling was nor what the case was even fully about.
> Imagine, for example, a case is about someone disclosing the location of nuclear arms to their spouse.
The location of most US nuclear weapons are public. Moreover, if you suspect that the location of some you intend to be secret may have been compromised then the obvious sensible response is "move them" rather than "keep even the legal theory of the case a secret forever".
This is why the primary ones whose location is intended to be a secret are on submarines -- submarines are easy to move around.
This example requires a secret ruling but not a secret interpretation of the law.
Why can't they? They can redact names of individuals in a case like that. "Oh some person who was married to some other person who works for some agency knows where nukes might be". That's not a risk.
They could even redact larger portions of the case and still release the interpretation in some form.