This is because as far as the new forum is concerned, every post
is a new post, since they were all imported from phpBB. But in truth there haven't been that many new posts since last week.
Your best bet is probably to read through the "unread posts since your last visit" page
https://www.supermanthroughtheages.com/smf/index.php?action=unreador the "most recent posts" page
https://www.supermanthroughtheages.com/smf/index.php?action=recentand after reading through the top few listed, you'll get down to posts that you've already read. Then hit the "Mark All Messages" as read button on the main page and logout.
You only have to do that once because of the migration.
The next time you log in, the "new post" indicators should be accurate.
Also note that the most recent posts are always listed at the bottom of the main page, whether you've read them or not.