You're using one (18-wheeler) truck for all of the different kinds of food for the last mile in terms of distribution center-to-supermarket, but one (pickup) truck each for the different sources for the farm-to-farmer's market. Not to mention that the distribution center is likely closer than the farms.

But food doesn't grow in distribution centers. At some point you have to transport the food out of the farm. That would result in the following chains (simplified):

Local: Farm -(1)-> Farmer's market -(4)-> Consumer's home

National: Farm -(1)-> Distribution center -(3)-> Supermarket -(4)-> Consumer's home

International: Farm -(1)-> Harbor/Distribution center of producing country -(2)-> Harbor/Distribution center of consuming country -(3)-> Supermarket -(4)-> Consumer's home

Where (1) is the farmer's pickup truck, (2) is a massive high-seas-capable container vessel, (3) is the 18-wheeler and (4) is the consumer's audi.

Yes, (2) and (3) may be vastly more efficient in CO2 per transported item - but it doesn't change the fact that they happen in addition to (1) and (4).

That's not the case. When you talk about large-scale farming, the (1) in the latter chains are actually (3).

I would note it's frequently an 18-wheeler from the farm to a distribution center as well.