Well, its a similar concept from differing domains, code editing, and tree viewing (commonly filesystems).
IME the former uses "fold/unfold" for the hiding of contiguous blocks of code whilst the latter uses "expand/collapse" for an information hierarchy. Certainly the underlying widgets do so:
http://www.scintilla.org/ScintillaDoc.html#Folding
https://developer.gnome.org/gtk3/stable/GtkTreeView.html#gtk-tree-view-expan...
Emacs, Qt creator use fold/unfold, Eclipse does not have folds on the text buffer, but shows the structure in a tree view in a separate pane and uses expand/collapse in that tree view to fold/unfold the text, yes it uses both terms to describe the action.
I think its fine for Geany to follow the historical usage and leave both terms.