IIRC, Glade-3.8 is the last series supporting GTK+-2.x, while Glade-3.10 is meant to be used by GTK+-3.x applications
You are right. I should have looked at the header of the glade file more carefully. It clearly states that the interface requires gtk+3.0.
So the preferred solution here would be to install glade-3.8, but on small projects like yours, simply running sed over the XML would also be an option.
The difficulty with sed is that it is a bit awkward to process multiple lines. The GtkVBox looks like this
<object class="GtkBox" id="box1">
i.e., one learns only in the fourth line that we have a vertical box. But all the same, it won't be hard to hack something to process the file.
EDIT: It seems that glade creates the proper hbox and vbox, if one adds it to the gui by right-clicking on it, and chooses "Add parent -> Horizontal box". Perhaps, this option remained in the code by accident...