Making a post to a forum could get seen by the wider public, but as an end user you knew that statistically seldom would the wide public do so, because a forum tended to be engaged with by specialized topic, and the wider public was seldom going to make the effort to enroll to see or respond to the content in a given one.

Like you could make a "public post" to a Pokémon forum knowing that most likely it is only getting seen and engaged with by other fans, and the context of that interaction would also be narrow in scope.

I liken web forums to tables at public events: you are technically able to be seen and heard by the general public, but in practicality most of the public will never notice you at the table nor overhear what you say nor bother to walk over.