This is true. I am a moderator for /r/mildlyinteresting and two weeks before we became a default, we were asked if we wanted to opt-out. It was hotly-debated between the moderators for about a week and any one of us could have answered the admin, but we put it to a vote and ended up becoming a default.

That image related to posts from that subreddit being seen in /r/all. Automoderater puts /r/all flair on such posts in certain subreddits.

That they can opt out does not mean they can opt in.