Based on the article, it sounds like he didn’t use a thumbs up emoji to accept contracts in the previous instances.

A thumbs up is definitely a dangerous reply in the same way that OK is, but people use it that way anyway. And while it was dumb on his part to be unclear, it was also dumb on the buyer’s part to interpret such an unclear answer as an acceptance.

As to not answering, people forget to respond all the time.