To be clear, it's not fair to classify them as publishers. Newspapers and magazines are not mediums of discussions like forums are, they're compilers of articles that people spend days to write and submit for approval.
To use an offline metaphor, forums are like clubs where people meetup in a room and have informal discussions about the topic of the club. Such clubs often have someone acting as a moderator, since if discussions derail from the club topic then the host of the club will become disinterested, as well as potential newcomers who have an interest in the club topic.
In my opinion, if a club must be classified as either a "platform" or a "publisher", I think the word "platform" fits better. However, you consider platforms as being limited to communication mediums like postal mail and phone networks. If that's the case, then I think clubs classify as neither.