When you enable mailing list moderation, and also allow external / non subscribers to send email, only messages that are coming from the non-subscribers will be moderated. Messages from subscribers are not moderated.
If you want to also moderate posts from your subscribers (i.e. moderate everything), you need to turn off this option:
"Do not restrict posts based on SENDER address"
"Others not rejected. For moderated lists, all posts are forwarded to moderators. The switch affects only moderated lists."