The othertwo threads have a light purple background in the forum index and a "Notice: This is a "Friends Only" Thread" message at the top when I open them.
This thread, however, seems to have neither of these things. (For the record, I'm not on Morrus' friends list right now.)
Yeah, it appears to have magically changed status by itself between [MENTION=177]Umbran[/MENTION]'s screenshot and yours, [MENTION=13]Darkness[/MENTION], while I was out at the cinema! You're a mod, though, so you'd see it anyway.
"Friends only" does now seem to be working. However, it's maybe worth noting that the threads seem to have the same "mouseover" issue as threads in Octopus Rock - the first few lines of the OP in a "friends only" thread can still be viewed by hovering the mouse on the thread title.