Unfortunately, this isn't possible with how the system is currently designed. Back when that was asked, the inbox worked very differently than it does today, and has been reworked a lot since then.
The moderator inbox is stored per site, not per user. When a message is generated for it, we store the site Id the message belongs to and the inbox pulls all messages belonging to the site. We don't store those messages individually for each moderator, and the user Id that sparked the message is not stored anywhere, so we cannot hide the message for individual users.
Messages are not marked as read on an individual basis. When you open either inbox, we simply update a DateTime field that indicates the last time you opened the inbox. All messages after that recorded time are displayed as "read" the next time you load the inbox. In order to mark the notification as read when you create a new meta post, we'd have to update that time, but that would also mark any other messages you might not have seen yet as read also, because it's an all or nothing deal.
Until such a time that someone decides we should record read status individually per inbox item instead of just following a date, there's nothing we could easily do to make these notifications less annoying.