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?