I think the personal historical context between the two parties is the important part here. Reading the article, I get the impression that this was not the first instance of these two conducting business in this way.

If the buyer has previous experience with the seller responding to a contract with a thumbs up and then processing to fulfill that contact, why wouldn't they interpret that as acceptance in this case?

To use your own analogy, it would be like a couple who regularly texts 🍌🍆🥒🌭🍑🌋💧🏔️ - 👍 to indicate sexy times having one party reinterpret that meaning suddenly when it's convenient for them.