Secondary sources are works of scholarship that analyze a historical period. I would not normally consider a sculpture to be a secondary source, as it is not generally speaking a work of scholarly analysis, but a work of artistic originality. Sculptures can be very important for understanding commemoration and historical memory, and for embodying artistic and historical narratives, but they are not works of historical scholarship, even if they are based on historical scholarship.

One could press the boundaries between art and historical scholarship, and some historians do highlight the places where it blurs. Documentary film, for example, definitely sits at that boundary, as can historical (non-documentary) films. And certainly there are digital works that deliberately play at that intersection. But I think extending that overly broadly to all forms of media would both make a hash of art and a hash of history. They are usually quite different things, even if they sometimes intersect and often inform one another.

I can imagine a very belabored, theory-heavy argument that one could try to make to draw them together but it strikes me as going in the face of the really obvious differences in levels of engagement. The kinds of narratives and analysis that sculptures participate in are very different than more didactic forms of speech like written prose or film or other media whose core are explicit linguistic expression. I suppose you could make a sculpture that lectures you on history (my mind goes to Disney's Mr. Lincoln animatronic), and I am well aware that the moment you suggest to an artist that something might not be art or capable in art they will come up with a clever exception (which I respect), but I think generally speaking one would not count a sculpture as a secondary source, but as a primary source (albeit from a different time than what it depicts — a sculpture of a Confederate general erected in the 1950s is more about the 1950s than it is the 1860s, in other words).

This is probably not the answer you would be expecting, but in art history, sculptures of historical events take a lot more time and ability to design than other types of artwork. Forty years, however, in most cases, would be the equivalent to a person's lifetime, so this might not be the case.

Usually, sculptures of historical events have a deeply strong connection to national sentiments and feelings, so it is not uncommon to find a sculpture or artwork in general that has a time delay. Sometimes, even, artworks are designed as a way to set in stone a memory of the past that had only been travelling around in books or other artistic mediums.

One of the most interesting pieces of research (and this will ultimately depend what kind of statue we're talking about here) you can conduct when artworks have an odd time difference is to look at the iconographical clues presented. It was common for small reliefs that circulated during the 16th and 17th century to copy another form of artwork—most of the times, it would be the engravings used to illustrate books. Some artists of smaller crafts would be very skilled in their ability, but so creative, and copying was one of the most frequently used techniques to overcome this difficulty. I'd say this statue could be one of these cases, but you'd have to conduct a full on investigation to figure it out, with no guarantees you would find the original source.