@mirkobruner Welcome to the forum, mirkobruner
Page 17 refers to the sea battle. The rule expresses that all present units there must participate in the sea battle. For example you cannot assign the planes to attack enemy sea units to keep the possibility to offshore bombard with your ships. So all units must participate in the sea battle.
Page 18 refers to the “complete” amphibious assault simply saying that the attacker has to choose which planes attack in the seazone and which planes attack in the territory, as they cannot do both.
So you are correct - the movement is fixed.
It is just that the rules you refer to address a different background.
Does that help?