To me the supply units seem to be more supply distribution points (administration and transport)than true supply depots (where the stuff is held before dispatch, and which also tends to be either city hexes or the map edge) so I'd decline your suggestion.
Unfortunately I only have Caporetto so I can't make a definite comparison.
They'd still have some stocks on hand, and they'd still take up space. But maybe you're right. It would certainly explain why it's not mentioned in the rules.