It is license specific, and if forking is not allowed then it is not an open source license [0]. I consider "Open source" and "FLOSS" as equivalent terms, since "open source" as a term was proposed as an equivalent term to avoid negative connotations of "free" as low quality or not worth paying for[1].
The L for “libre” in FLOSS may be to avoid negative connotations of "free" as low quality or not worth paying for.
The link you referenced are talking about an entirely different conflict, and it is very much not meant to be equivalent. Open Source was all about distancing from the political ideas associated with free software and the GNU project.
[0] https://en.wikipedia.org/wiki/Free_and_open-source_software#...
[1] https://web.archive.org/web/20021001164015/http://www.openso...