I have tested this and confirm that the dismiss button clears the unreadable notifications.
That said, we are bound to confuse already frustrated users as they might click the notification (the mention) before using the Dismiss button, either because they haven’t read our instructions yet on why were mentioned, or just because they need some handholding.
Well, at least there’s a workaround: some users have accumulated quite a few of these and will be happy to have that reset. We’ll be upgrading to 3.0 tomorrow and we’ll try to help them afterwards.