How would this be zero sum?
Since there already was enough food, the market is saturated, so it would effectively reduce the price of all food. This would change the ratio so that the farmer who grows more gets more money in total, and every other farmer gets a bit less.
As long as there is any sort of growth involved - more people, more appetite, whatever, it would be value creation. But without growth, it's not.
At least not in the economical sense. Saving resources and effort that goes into producing things is great for society, on paper. But with the economic system that got us this far, we have no real mechanism for distributing the gains. So we get over supplying producers fighting over limited demand.
The world is several orders of magnitude more complex than that example, of course. But that's the basic idea.
That said, I'm not exactly an economist, and considering it's a bleak opinion to hold, I'd like to learn something based on which I could change it.
About the farmer needing to change jobs, in the interview that is the subject of this thread Ilya Sutskever speaks with wonder about humans' ability to generalize their intelligence across different domains with very little training. Cheaper food prices could mean people eat out or order-in more and then some ex-farmers might enter restaurant or food preparation businesses. People would still be getting wealthier, even without the tailwind of a growing population.
And then there's followup needs, such as "if I need to get somewhere to have a social life, I have a need for transportation following from that". A long chain of such follow-up needs gives us agile consultants and what not, but one can usually follow it back to the source need by following the money.
Startup folks like to highlight how they "create value", they added something to the world that wasn't there before and they get to collect the cash for it.
But assuming that population growth will eventually stagnate, I find it hard to not ultimately see it all as a zero sum game. Limited people with limited time and money, that's limited demand. What companies ultimately do, is fight for each other for that. And when the winners emerge and the dust settles, supply can go down to meet the demand.