Can you point out where it says this? I need to correct this with s/topic/course as all the content and code is available for free and licensed as AGPL. Even API access is available to build alternative clients.

This is supposed to refer to "courses" which are structured, project-based peer groups where mentors spend time to evaluate your projects and answer your questions. By compensating them for their time, we can get really high-quality mentors for 1-1 feedback on your learning projects.

The unstructured peer groups (in the Slack channel) are and will remain completely free.