The "Opening Move" before turn one is a CP move only. It is into Belgium only. See 16.1 for details.
The playbook actually states: "The Opening Move must include an invasion of Belgium."
I was under the belief it is possible to move other SPs in Germany as well as into Belgium. Therefore possible to bolster the south. The 'include' suggests Belgium AND Germany / Luxembourg are permitted ( but not France as that is clearly stated further down the paragraph)