No, it was inside.
$topicOptions['id'] has the topic ID but I'm not sure we can determine whether it's a new topic in create_post_after, though. It can be done in create_post_before by testing for the topic ID (empty), but not after -- I think. It may be smart to add &$new_topic as a parameter in the create_post_after (and in create_post_before, in case you want to 'cancel' the new topic status and redirect the post to an existing topic.)
I'll do just that. (Actually, done.) (Just needs to be committed.)