To be clear, the wrapping options are already implemented in Scintilla, but Geany does not have any way for the user to select the option. So all you get is word wrap or no wrap.
I guess whoever made the UI to turn wrapping on only wanted it for documentation (ie text) not for code. Long lines are often frowned on in code, and since there is no way of knowing where a wrap is gonna be (since it depends on how wide the screen is), two users looking at the same file may not see the same thing.
Though I suspect that no matter what type of wrapping is chosen tabs and wraps may not interact well together.