he had done at least four other contracts with Achter via text. He said the only difference this time was Achter responded with a “thumbs-up” emoji instead of “ok”, “yup” or “looks good.”

I think this precedent set between the two parties is relevant, giving an otherwise casual text message a little more authority. Not that one word text responses are how you should enter a contract, but it’s what they had already been doing.