To be clear, do you mean you:
 * think it's not better, maybe even worse
 * think it's not _much_ better (but still _slightly_ better)
 * are undecided
 * or something else
?
Also, "guix build -S" returns the source code (after snippet / patch,
if any), not its derivation. For the latter: "guix build -S -d"

FWIW I don't think mentioning patch-and-repack is too helpful here
either.  Also, I'd like to use consistent wording at least within this
section, so here "source" means "upstream source" whereas "source
derivation" is a shorthand for the stuff Guix builds.  Yes, the
derivation is not the same thing as the output, but I again fail to see
how being overly precise is helpful.  That being said, I'm open to
suggestions.

I am not reading an answer to my question.

I don't think I've mentioned patch-and-repack (at least not by name, which you seem to be referring to?).

I would not recommend "source = upstream source", as the more general meaning is used in (guix)Introduction and <https://www.gnu.org/philosophy/free-sw.html> and elsewhere, otherwise terminology would become inconsistent, which can lead to misinterpretations.

I don't think there's such a thing as 'overly precise'.

My suggestion is the same as your suggestion:

You could s/source derivation/the result of
@code{guix build -S}/, but I don't think that's much better.