I went to university ~'94, and we were certainly told in no uncertain terms that we were expected to cite anything we copied, code not an exception.

I don't think it means if you call SDL_CreateWindow, if that's what you mean with "on a certain line of code", but if you were to for example implement a window creation function by copying the contents of SDL_CreateWindow and adapting it.

EDIT: Citing everything is extra important when working on projects for courses, because even when its overzealous in terms of what is legal the point of doing the work is to show that you understand the concepts properly, and then its important for whomever going through your code to be able to tell which bits you actually worked on.

I don’t see that text in the parent comment or in either of the three linked pages. Where are you referring to?

It's on http://graphics.cs.cmu.edu/courses/15-466-f18/ linked in the response further down, in the "Don't steal" section.