Generally speaking, shinshoku are certified by one of the shintô associations, a particular jinja, or a university, as competent to perform a shintô ritual, and/or serve in or run a jinja.

Mind you this is a certification of competence, not a license to practice. (Licenses are issued by a government; there are separation of religion and state issues when a state gets involved in licensing religion.)

minzoku shintô has a different certification process. Instead of top-down like the jinja model, it uses a bottom-up model where a person's competence is judged by their local community.

In other words, if your local community accepts your ability to perform, serve or maintain a shrine, you are in their eyes a "legitimate" shintô practitioner, and qualified to perform their rituals. So while it may not be jinja shintô, it still is really shintô.