If you read the article, this farmer had accepted contracts from the buyer through text multiple times. The only difference this time was the thumbs up vs. saying ok.
That and he could get $41 / bushel instead of the $17 that was in the contract.
Edit: let my flesh is this out a little more.
In my opinion, he was trying to back out of the contract and thought he could based on a technicality. That he didn't physically sign it. But he didn't physically sign the others and still accepted and delivered those.
So? Even if he did work before, he didn't sign a contract.
He should be free to change any deals that don't have a notorized signature attached to them.
This is basic business. Hopefully he won't get convicted and future businessmen can learn from this.
Contract law is interesting here. There's something called implied consent that makes the previous events very relevant - "Implied acceptance occurs when the parties act in a way that indicates their agreement with the contract terms. It also occurs when a product or service is purchased; payment for the item in question is considered implied acceptance.". This can be verbal or nonverbal.
So since he acted in a way that indicated acceptance previously, the form of acceptance (single text, positive reaction) it's implied he was accepting. A hand shake also constitutes acceptance if it was face to face.