You know in an interview loop where you consider "culture fit?" That's it. That's the conspiracy.
For a journalism job at the Telegraph you're going to need to have a resume where you've written the right kinds of articles with the right kind of tone. Have a bunch of articles that you've written sympathetic to Corbyn and those around him and you ain't getting the job.
They won't even need to police you that hard once you've gotten the job, because you've spent 20+ years being brought up a certain way and you're probably not going to massively deviate from your past all of a sudden, particularly not if you're clearly getting paid well based on your current trajectory. Once you get vetted in the interview process they know what they're getting based on the way you present yourself, the way you talk, your background, where you grew up, the schools you went to, etc. And when you've been there long enough you'll vet other candidates the same way.
And its largely the same kind of interpersonal mechanics that govern the formation of cliques in high school, enforced by interviews and performance reviews, with large incentives where if you conform you climb the ladder and gain more wealth and power. Nobody needs to write it all down just like there's no written rules that he art nerds tend to hang out with the art nerds (and I'm sure there's examples of people who you think you can point to who broke the molds there, but they're notable precisely because they broke the molds).