Its whole premise is ugly.
When you read a topic, the id of the last message you read (if you're part way through a topic) or another suitably high message id (last posted forumwide) is used to track the 'last' one you've read.
All the mod does is enforce that the value that goes into that column is the highest possible message id, so it will never appear to have new posts since as far as SMF is concerned, the last post you read had id 2147000000 or so (largest number that can be safely used in 32 bit PHP)
Marking it unread will break this behaviour, too, because it'll reset the value of that column, however if you're marking it unread presumably you're unignoring it anyway.