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.

The highlighted part is only 15 words. Why stop reading after 7?

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