The Shuowen indeed already mentions sounds. But the fact that there are sound components isn't contradictory with them being also semantic.
> I don't think we need to look too much about what ancient scholars thought about characters
Let's say that I enjoy the study of ancient thought processes, and that some of it seems to be recorded in etymology & the like.
> were not trained to do science
I believe their approaches were different, more "naive" perhaps, which might be the source of confusion. Let me give you and example where we currently fall short, and where they did not.
Consider political organisation: from Plato (The Republic) we knew that they studied the way one political system changes to another, and studied the causes of this evolution. Yet, the overwhelming majority of contemporary people have been "in-doctrinated" to think that democracy is paramount.
Not only are we disregarding history and previous data, but most businesses, where we all spend a considerable amount of time, for subsistence, are fundamentally non-democratic: we're also severely inconsistent.
Perhaps ancient Greeks didn't had modern means (e.g. peer-reviewed papers, social "sciences"), but they nevertheless captured the jest of it, while we're essentially not.