But here the meaning of "rule" is not "straightedge", but rather the derived meaning "a thin printed line or dash". So "ruler" would be improper because that word doesn't have the typographical meaning.
Wait, so the "ruler guides" are misnamed, they are just "rules"?
In any case, the things they added could very well have been called "column-divider" and "row-divider" with much less ambiguity because not everyone who has to wrangle with CSS is a designer by profession or by choice.
In page layout software, the thin UI elements bordering the left side and top side of the page, with the little tick marks, is called a ruler. The tick marks on the ruler are called rules (just like the rules on a physical ruler used for measuring things). When you click/drag on the ruler elements, you create guides (or guidelines).
I’ve never seen “ruler guides” verbatim, but I would take that to be shorthand for “guides one could create using the ruler” (which would be a mouthful) to disambiguate the word “guide” when there isn’t sufficient context for the reader to likely understand what was meant.
"ruler guide" - One of those English noun chains that my translators hate so much. Because in most other languages you actually do have to say "the guide of the ruler" or some comparably awkward mouthful.
In any case, the things they added could very well have been called "column-divider" and "row-divider" with much less ambiguity because not everyone who has to wrangle with CSS is a designer by profession or by choice.