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?

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.

I still disagree with arbitrary, just less strongly.

Whether his opinion on GPL is relevant, or if it is, how important it is, is up for debate. But I still don’t think it’s “based on a random choice or personal whim, rather than any reason or system”.

You’re referring to a different kind of dispute than I (and parent) was talking about. In this case, Stallman did handle the dispute about whether GNU Parallel’s citation notice conflicted with the terms of the GPL.