> Or because the structure is used outside of linked list contexts

That use case is possible with the particular implementation that is now used by curl:

    struct fileinfo {
      struct curl_fileinfo info;
      struct curl_llist_element list;
    };

You can use the payload (struct curl_fileinfo) outside of a list without incurring any overhead.

I think one of your other points is more likely, or simply "it was fast enough and we went for more interesting features than for such optimizations".