The Popular Mechanix article says the message will stay in view 'at least two hours', while the Getty Images caption says 'up to two hours'. By the looks of it the latter seems more likely - pump enough coins into the machine and you can make earlier messages roll out of view!
This is a message to location service. There is no broadcasting to all Notificators in London simultaneously, and each requires a coin for each message sent. The message is only up for two hours, and is not tied to a user. The intended purpose was for a specific person to be present at the destination.
How is this analogous to twitter any more than say, the classifieds of a newspaper?
You are totally right. Also, here you are writing on a piece of paper instead of typing in a text box. The viewing interface is also totally different as one is a either a monitor or a mobile device, and another is a crude bulletin board thing. And don't even get me started on ...
Twitter missed that part :)