It is not true, I have created virtual panels in GLADE 3 and later, I have been able to edit them with GLADE 3, like what I am showing in GLADE 3, which were created with GLADE 2, the only thing is that some wigets have been decapitated.
GLADE-GTK2 IS GLADE 3
GLADE-GTK3 is GLADE version above 3.8.6
Edit: You can open a glade gtk2 file with glade gtk3 these days. They must have added support in recent versions because horizontal and vertical boxes were depricated in earlier versions of GTK3 so that I take back....
but the main point here was you need linuxcnc's gladevcp environment to open a file with gladevcp widgets or else glade-gtk2 can't open it at all and glade-gtk3 just shows dead widgets