If I give Apple Mail my credentials for my GMail account, I would expect Apple Mail to be able to access my email in my GMail account. Switching the word "email" to "DM" doesn't feel like a meaningful difference: if I'm using a third-party client to access and send messages, of course the third-party has access to my messages. Would I expect Tweetbot to be unable to access any tweets other than the ones sent from Tweetbot? That's... not a very useful third-party client. These were third-party Messenger clients; they had access to your Messenger DMs if you opted into using them.