An organization whose members may not disclose the purposes, practices, or activities of their society to civil or ecclesiastical authorities Parse carefully. There is legitimate need for secrecy that the Church recognizes and endorses. The confessional secrecy, military secrets, trade secrets etc. are all examples of secrecy that are not condemned, and at least in the case of confessional secrecy are vigorously supported by the Church against all non-ecclesial intrusion.
There seem to be three parts to the ban:
- it has to be an organization, and not an individual or an ad-hoc temporal group. For example, it is not sinful for a Catholic to form a group which purpose is to study Chinese pottery (or turnip agriculture) and not tell anyone.
- it has to be kept secret from the church authority AND from civil authority. For example, the CIA is a very secretive organization, but it is supposed to disclose anything to the proper civil authority.
- it has to be the fundamentals of the group; the phrasing seems to allow for some specifics to be kept in general secrecy, for example, as secrets in science or trade. We don't know the Coca-Cola recipe, but we know Coca Cola's purpose, practices and activities in general. In principle, the courts might compel to disclose even the recipe to the court, in a context of a lawsuit.
Great analogies to legimate secrets.
The Seal of Confession is definitely one as are the trade and CIA secrets totally legimate.