Could you please more clearly distinguish "intertextuality" from "building upon previous work", which is central to academics, and is a far older concept than "intertextuality"?

Indeed, isn't "intertextuality" at best building upon the previous work of the idea of "building upon previous work"? Nowhere in the idea of standing on the shoulders of giants does it say you need to know the giants' names or even how many there are.

For a while I had thought that the term was specifically referencing the unintentional part, though this raises the rather difficult prospect of telling people who are thinking deeply about something that their output is of an unintentional nature. But then you mentioned analytic philosophers heavily citing each other, which is very deliberate indeed.

So at this point, I'm uncertain what you're getting at. Yes, people build on previous work; sometimes they hardly even realize what their building on, and sometimes they cite it heavily. And...?