I've established a supply line between two settlements, one has a lot of items in the workshop, the other only has a few. The settler travels the route along with a packed Brahman bull. The supply line is visible on the map. But the inventory of the workshop in the settlement with only a few items STILL only has those few items.
The inventories are not sharing. This needs to be fixed. Otherwise this perk was wasted or could have been chosen later on.