No, sir, there's no DMs! "Private mentions" are just simply posts that is available to some specific people mentioned in!
Look at Pleroma. Pleroma have DMs implemented, just similar to Twitter's (to have full experience you will have to install/try Pleroma with Soapbox FE installed, like seal.cafe ) and Mastodon's "DMs" are NOTHING compared to it.
Eugen have to develop "DMs", the real ones this time, similar to Twitter's, Pleroma's (+Soapbox's too!).
cc: @arunshah240 @Gargron