Solved it.
Either use gtk_binding_entry_remove()
to unbind Ctrl-F
on the TreeView, or use
something along the following lines:
css_provider = Gtk.CssProvider()
css_provider.load_from_data("""
@binding-set unbind_search { unbind "<Control>f"; }
GtkTreeView { gtk-key-bindings: unbind_search; }
""".encode('utf-8'))
context = Gtk.StyleContext()
context.add_provider_for_screen(
Gdk.Screen.get_default(), css_provider, Gtk.STYLE_PROVIDER_PRIORITY_USER
)