In context of adding burden of having someone to translate for you in Japanese, the statement he made seems good enough and to the point.

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 mean what other reason would there be a reason to bring up gnuplot, if not for it's assumed connection to GNU?

I guess "GNU, but gnuplot is not GNU" could be one, but perhaps at that hectic moment RMS didn't consider that option!

> "In case you just said gnuplot is part of GNU,” in front of it is not too long

No longer than, say, adding GNU/to the front of GNU/Linux.