I'd perhaps generalize that to "it's useful to have visual grouping correlate with semantic grouping"; applies to separating words with spaces, sentences with punctuation, paragraphs with newlines, lines of code, UIs, and much more.
An important question for this also is what qualifies for "single thing"; you can look at a "for (int i = 0; i < n; i++) sum += arr[i]" as like 5 or more things (separate parts), 2 things (loop, body), or just one thing ("sum"). What array languages enable is squeezing quite a bit into the space of "single thing" (though I personally don't go as far as the popular k examples of ultra-terseness).
of course, since C is a low level language, implementation details seep in there but those are the 2 major logical concepts that mentally can be quickly parsed as a single unit each.
Some languages allow reductions on collections, so the actual iteration becomes logically part of that single unit, so it might count as one, but I'd say that's the absolute upper bound of complexity you should stuff into a singlwe line.
The goal here is to make every statement conform to a template so that your brain quickly recognizes it and you can move on, rather than you having to break it apart bit by bit to figure out the goal.
An important question for this also is what qualifies for "single thing"; you can look at a "for (int i = 0; i < n; i++) sum += arr[i]" as like 5 or more things (separate parts), 2 things (loop, body), or just one thing ("sum"). What array languages enable is squeezing quite a bit into the space of "single thing" (though I personally don't go as far as the popular k examples of ultra-terseness).