You’re government agency, and you ask your lawyers about how the law operates given a particular set of circumstances.

In response, the lawyers provide you with advice about how they think the law applies to this situation, and what findings they think a court would make given the evidence etc available.

Information subject to attorney-client privilege (as this information would be) is exempt from release under FOIA anyway. Dropping a security classification on top adds protections against unauthorised release by insiders.

Sometimes there are good reasons for this, such as to protect particular technical capabilities that are easily circumvented if adversaries know that they exist, or to protect human sources who might end up dead if found out. Other times, there are no good reasons to do this but without knowing all the circumstances it’s impossible to tell either way.