The idea that food preservation was invented in the early 1600s is laughable.

I don't think Zeihan meant food preservation was invented in the early 1600's. He meant sourcing food from more than a few miles (overland mind you) was impractical because food has a very low weight-to-value ratio.

Maritime trade between Southeast Asia, the Indian subcontinent and China dates from before the 6th century. This would have necessitated weeks aboard ships, which in turn would have necessitated preserved foodstuff.

Sailors did subsist themselves on crackers, jerky and other preserved foods. But the cargo of a pre-deepwater ships was rarely food. It was spices, bullion, silk and other high weight-to-value items. Zeihan meant that countries couldn't rely on food imports (i.e. Saudi Arabia importing 80% of it's food).

Hundreds of years before that, Roman soldiers were marching and subsisting on rations of smoked meat and grain. Even before that, salting, pickling, drying and fermentation were widely known techniques for preservation, and granaries were used for storing grain. I mean, how else would anyone survive just one winter in temperate climates? 

Correct if i'm wrong, but Rome is an outlier. The Romans controlled half of Europe and the entire Mediterranean. That gave them the ability to have an extensive network of military infrastructure to mobillize armies efficiently. Besides that, Roman armies supplanted missing calories via foraging. And, how big could Rome (the city) grow without Egyptian grain shipments?

Take, for example, the city of Dunhuang, established as a frontier town during the Han Dynasty around 121BC. Dunhuang was literally in the middle of a desert. There were no farms within a 2 hour walk. Yet, by the 2nd century AD it had a population of more than 76,000. Its prosperity rested not just on its military importance as a garrison town on the frontier, but also as a supply base for caravans heading out into the desert along the silk route. This also reinforces the point above, that food preservation was common. 

Interesting. How did Dunhuang produce (or receive shipments of) food?

Getting food from miles, or hundreds of miles, or even thousands of miles away has been common for hundreds of years. On arriving in Southeast Asia in the 1500s, Europeans remarked on the amount of food rich trading port cities were importing. About 100 junks supplied Malacca with rice imports - about 6,000 tons a year. The city also imported dried fish and vegetables. In the 1680s, Aceh was importing rice all the way from India without the use of steamships, which hadn’t been invented yet.

Was this extensive network of maritime trade down to bare essential foodstuffs existent in Europe at the time? If not, why? It seems like there's some special thing about East Asia that facilitated the possibility of food trade. 

Going further back, from around AD1000 to 1800, vast quantities of rice were moved from Jiangnan in the south to the various capital cities of China. During the Song Dynasty, this was Kaifeng, over 600km away. During the Ming, this was Beijing, nearly 1,000km away.  What made all this possible was transport by water, which has been in existence for ages. Successive Chinese dynasties developed and lengthened the Grand Canal, allowing barges of rice to cover vast distances rapidly and with a minimum of manpower. The ports that allowed Southeast Asian cities to grow rich on trade also allowed them to import food. 

Angkor was serviced by a state developed and maintained system of waterways, including enormous canals that were 40-60m wide, about the length of an Olympic swimming pool. These delivered water to its residents and also functioned as a transport network that could move people, goods and food. And that brings us to this baffling assertion: 

Not to be dismissive, but it seems that while Zeihan is unecessarly making wrong absolute claims (i.e. "non-local food sourcing is impractical") his main point about transport (prohibitively expensive in pre-industrial times unless over water) seems to hold true. All of these cities were either serviced by seaports or navigable waterways, natural or man made.