Just retested, now I consistently get no result. I observed the copy behavior described in the report before, so maybe due to Gtk update in between.
Background is that zim handles the drop itself, but not the drag start. Looks like the view takes a shortcut on deciding what data type to drag and does not check our advertised data types.
The copy behavior probably is the case where we do get a compatible data type and paste it, but we should either do the delete ourselves or send some signal to notify that the delete can happen.