Do those movement and supply rules apply to both players?
Supply is normally drawn through friendly areas, a friendly area is the one last moved through by your troops. If I use the movement from Antwerp to Aachen as the Entente does that make the Netherlands "friendly" to me for the purposes of drawing supply?
The Netherlands is not a playable area, so it's neither friendly nor enemy. Think of it as not existing in play; not part of the equation at all. Treat Antwerp as being adjacent to Aachen and Köln. The answer to your question becomes clear then.
This point is fairly hypothetical though, if the Entente are crashing into Germany the game is all but over anyway.