It appears that this is being done with official blessing, but I still don't see how it's supposed to work.
Essentially, they are promising one-for-one convertibility with U.S. currency. Bermuda already does this. You can pay in U.S. currency or Bermuda dollars in Bermuda. The qualification is that the Zimbabwe currency will not be exportable. You cannot spend it outside of Zimbabwe. It pumps a little bit of hard (by Zimbabwe standards) currency into the economy without the immediate danger that it will seek shelter outside of Zimbabwe.
Not a terrible idea. Zimbabwe stopped issuing currency about ten years ago. They literally could not get enough hard currency to pay printing costs. Transactions are in U.S., British and Kenyan currency these days.