I don't believe the original report of this "new" hatchet exploit was ever legitimate in the first place. There was only one report of it on the official forums, which was closed by Kay (I think) asking the OP to send the details on how to reproduce it. That was a while ago.
Fast forward to now, and we have all the new/upcoming

, and the only exploit addressed is the animation cancellation via "Aimed Throw". There is no mention of them fixing any other exploit with the hatchet.
I think most likely the person who reported this may have either *thought* the damage was doubled when it was not, or it was a one off that they were unable to reproduce.
Either way, I don't think there is a currently known hatchet damage exploit at the moment.