So there are ~6500 journalists, and about 400 (about 6%) of the ones who consider themselves left-leaning joined a club. It dissolved almost 15 years ago and got replaced with another group that has less than 200 members.

If that's your idea of a massive conspiracy, I don't know what to say, other than thank you for affirming that 'the media' does not describe anything like a monolithic group of people with shared ideology (aside, from, you know, thinking it is important to report the news).