1. Conclude that its 90% likely to be what happened.
2. Not burden someone by asking a question you don’t need the answer to.
3. Disclaim credit for work you don’t lead.
Maybe you are far too pre-assumtpive that there was any ill effect, or even ill intent, in the statement being made, especially if the same fact is accounted for in gnuplot FAQ.
I guess "GNU, but gnuplot is not GNU" could be one, but perhaps at that hectic moment RMS didn't consider that option!
No longer than, say, adding GNU/to the front of GNU/Linux.
It is a cautionary tale: never assume that because someone doesn't speak your language, he won't get it if you are bad mouthing him.
Here "blah blah, Stallman, blah GNU blah blah gnuplot" is enough for some not skilled in blah to understand what is going on.