Sure, some get through.
Of the streams that have the "No spammy watermarks" Submission Rule (fun, politics, repost, sports, gaming, reactiongifs, cats), some of those have Stream Settings for Moderation with "Automatically approve posts that appear to be safe-for-work".
Some submissions that don't meet the Submission Rules for a particular stream with that setting get auto-approved without a Moderator ever seeing them.
If users see featured image pages that don't meet the rules for that stream, they are free to use the image flag and a Moderator of that particular stream will take a look.