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).
CO2 for meat from NZ: Shipping + last mile
CO2 for locally produced meat: last mile
How is meat from NZ producing less CO2 than locally produced?