The actual tab widget has changed with your code. Remember you are using a custom function to create a notebook page tab with a close button. If you look at the add_page function in mainwindow.cc you'll see how the initial label is added.
you need to get a pointer/reference to that label widget to change the text. Something like this may work:
Gtk::Label *label = Gtk::manage(new Gtk::Label(text));
I'm not sure if this is the correct code but it is the general idea.
Gtk::Widget& widget = *m_Notebook.get_nth_page(m_Notebook.get_current_page());
Gtk::Widget label = widget.get_tab_label(widget);