This is done to not have the device combo box be too small in width when making the main sizer fit into the window. Not fitting the sizer would alternatively break Hidpi so it was best to just add an empty sizer to workaround this problem.
This is more logical as the mic is plugged into an EXI slot so it should be configured via the GameCube config dialog. This also allows to pass the right port number for the new dialog.