I'm pretty sure I had this issue - changed a bunch of my privacy settings, and tried to go find specific posts that were in fact DM's from the past, publicly displayed on my timeline, but couldn't. Also saw this response from FB on TC:

A spokesperson told Constine:

“Every report we’ve seen, we’ve gone back and checked. We haven’t seen one report that’s been confirmed [of a private message being exposed]. A lot of the confusion is because before 2009 there were no likes and no comments on wall posts. People went back and forth with wall posts instead of having a conversation [in the comments of single wall post.]“

Previously Facebook issued the statement:

“A small number of users raised concerns after what they mistakenly believed to be private messages appeared on their Timeline. Our engineers investigated these reports and found that the messages were older wall posts that had always been visible on the users’ profile pages. Facebook is satisfied that there has been no breach of user privacy.”