It looks like Guy is tweeting via TweetDeck. One could safely assume that he is actually at a computer tweeting (could be TweetDeck for iPhone or perhaps on an iPad). If you wanted to reach him via an @ reply or a Direct Message (DM) it’s a good idea to be able to tell when he in online and watching Twitter.
He probably gets hundreds of @ replies and Direct Messages a day. So, if he isn’t watching as your messages comes in, it will probably get pushed to the bottom in short order.
Lesson: Send your message at a time when it will be read.