Read https://help.medium.com/hc/en-us/articles/360018677974 section What does the checkbox do
The average article we publish does 10k views, which for most authors is way more than they'd get self-hosting and posting out to Twitter/FB.
I think also that Medium is still overly dominated by user generated content, i.e. what people here are calling low quality articles. You'd think of that content differently if it were better curated and better edited.
But all of Medium's subscriber revenue gets reinvested in their content. Most of that is author payments, but also they pay copy editors and curation editors. What they haven't done in programming yet is pay for any articles that have had a proper high quality edit. In programming that's a story editor, tech reviewers and a bug/errata updating process. But I suspect that will change because the subscription numbers are starting to be big enough that they can afford to invest in these things.
That's an official publication of the company behind medium. Maybe they had some bugs in the past or the people who complained or the author just made mistakes.
Try to access this article: https://medium.com/@rochus.keller/implementing-call-by-refer...
Can you access it or not?
If you hit the preview limit, you get the message "Not every story on Medium is free, like this one. Become a member to get unlimited access and support the voices you want to hear more from."
Non-metered (i.e. free) content is not behind the metered paywall. Also if the author posts a "friend link" the article is freely accessible, even if metered otherwise.
Do you have other sources?