What you seem to be missing is you're thinking in terms of the 'gold standard' of contract law, which is notarized signatures. But, these have never been required in the US or anywhere else. They're just used to make sure that people can't weasel out of shit with a good lawyer. That vast majority of business is conducted in an 'informal' manner. Agreements made with handshakes, verbal agreements, text messages, hell, even smoke signals would be perfectly legal if both parties were using them. What the judge was ruling on was, in 'informal' contract law, what the meaning of the thumbs up emoji meant in this context. And what it means can be two fold, depending on circumstances. Obviously, it's not being used as a means of encouragement here as that would be irrelevant to the conversation so that means that it's an acknowledgement, the equivalent of saying 'yes' in a legal context. Now, we don't have the benefit of the full conversation and the judge does, it's obvious the judge felt that the 'yes' was accepting the contract in this instance. Likely also based on the farmers past behavior as well. In short, what this looks like is the farmer agreed to deliver it, like he had in the past, but the prices changed and he didn't like the deal anymore and wanted to weasel out of it.
You'll find that people who routinely try to use this use informal, vague, language during contract negotiations or hard to record agreements are often the kind of people who try to weasel out of contracts when they're not to their benefit anymore when it comes time to pay up. Having unfortunately worked with a couple real scummy people in the past, I've gotten used to their modus operandi. In addition of always behaving as if you're going to stab them in the back (because they're planning on doing it to you, it's just a matter of when), they try to make sure that there's no way to call them on anything they've said or agreed to. They make sure there are no written records, or if there are they are incomprehensible, there are no witnesses, and there are no recordings of anything they say. It's why courts are so reluctant to let them out of a contract unless there's a damn good reason. And the fact the 'thumbs up' is vague is nowhere a good enough reason to let someone out of a contract. If they did that, everyone would use it to weasel out of everything the could and they know it! Might that shore up contract law? Maybe. But, even if it did, a lot of innocent people would be taken advantage of first before that happened.