It should have been obvious to Roko that either it was a worthless idea (in which case he was safe to post it... but shouldn't, because it's worthless); the other case was that it was a serious, valid idea, in which case he obviously should not post it.
See "Belief in belief", which is right near the beginning of the first sequence....
http://lesswrong.com/lw/i4/belief_in_belief/
It should have been obvious to Roko that either it was a worthless idea (in which case he was safe to post it... but shouldn't, because it's worthless); the other case was that it was a serious, valid idea, in which case he obviously should not post it.