To be precise, the mod works this way : when the display list is builded (index, viewforum), the already marked as unread topics are added to the new topics based on the last visit date, then the list is destroyed and recreated with the selected topics, then saved. When a topic is readed, or marked as readed, it is removed from the list of unread topics, and the list is saved. The same thing is applied for guests, as a last visit date from this computer is added to the cookie as a permanent data.
If you don't want to use cookie, you can use the database in place : part 5 of the mod : except for guests who will remain using the cookie, the topics list will be store in the users table (in a compressed way
), making the mod non sensitive to the computer/browser you are using.