Yes, during the initial launch of the Narrative Beta, all content will be posted to the Niches they are affiliated with when you create a Post. Niche owners will be the de facto Moderators and can reject the post to the Niche if it does not align with the Niche definition. The content creator will be notified via email from the system. In the future, Elected Moderators will be able to do pre- or post-publishing moderation. In that instance, the content may not be posted until the Moderator approves.
The only exception is when you are a Publication writer and you submit your post to both the Publication and Niches. In this case, the post is not immediately shared with the Niches since the Publication ownership has not been fully established. If the Publication accepts the post, when it is approved, the post will appear in both the Publication and the Niches. If for some reason the Publication editor does not accept your post for it's Publication, the post is returned to the writer's "draft" (see Avatar/Manage Account/Posts/Draft) so the writer can decide whether they want to publish the post on their own.