For metered content, "non-members get a select number of free stories in their member preview, which replenishes on the first of each month." (see https://help.medium.com/hc/en-us/articles/360017581433-About...).

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?

I'll keep an eye out for the exact message. I tend to avoid Medium as much as possible - scrolling is incredibly slow, and I recently found out medium storing several hundred MB of garbage in my localstorage, which it adds to every time I scroll.