I’m not sure I understand what you’re asking. Why is the intention of the author of the license in question?

In this case, Stallman simply clarified that Parallel’s notice did not count as a legal requirement and does not conflict with the GPL. His opinion wasn’t necessary, but since he wrote the license, it is authoritative. In this case, the question wasn’t brought to court, it was simply a clarifying discussion, and thus his intention did affect how things go in practice.

> And specially someone who is neither licensee or licensor?

Also wasn’t Stallman effectively the licensor or representing the licensor at the time, as president of FSF, head of the GNU project, and author of the GPL?

>His opinion wasn’t necessary, but since he wrote the license, it is authoritative.

No it isn't. Licences, like most legal documents, are construed objectively. The subjective intention of the author is totally irrelevant to the meaning.

You might have misunderstood what I said. It’s not up for debate whether RMS’s opinions or intent on the GPL have affected industry practice; that’s a fact of history. His statements on the GPL are authoritative in the sense that they may have prevented the courts from examining this question.

"Authoritative" has a particular meaning. You might have intended to say "influential". You didn't. I can only reply based on what you said.

What meaning are you thinking of, exactly? I looked up the definition and it matches what I intended to say in every dictionary I checked (Merriam Webster, Oxford, Cambridge, Dictionary.com…) Some of the definitions seem more or less synonymous with “influential”, maybe you’re making some incorrect assumptions?

RMS' comments and interpretations of the GPL have generally influenced how the industry deals with the GPL, and to the extent it has become de facto standard practice, the courts might take notice and take it into consideration.

Generally though, the contents of the text and (if applicable) case law surrounding its interpretation is more important.

the difference between law and code is the interpreter: law is interpreted by humans, significantly based on perceived intent, and code is interpreted by computers, which are expected to only act literally and deterministically.