Those aren't implicit because the "public interface" is the Object List, not the Data Structure List.

    struct list {
      float node;
      struct list *next;
    }

Above is the data structure; it implies operations. Below is an interface (class, in the article); it implies data.

    #define LIST_H
    
    float index(struct list *ls, int i);
    int find(struct list *ls, float x);
    void sort(struct list *ls);