Another thing that I'm currently cannot find out, when I create a Threadmark Index on a thread with multiple Threadmarks (sometime only one category, sometimes multiple categories), the "threadmark_index_threadmarks_list" does not show most of the time. I have tested on different nodes with different users, but it looks pretty random so I can't see the logic why it works (or not).
When I set some debug info in the template to output $threadmarkAuthors.count() and $threadmarks.count(), it shows 0 on both, even if on the threadmarkListingHeader it shows a number larger then 0 for Threadmarks (and thus also at least one author).