It actually doesn't say that it will be a Targaryen. It says has the blood of the Dragon. Which is I think him killing Danni. He fits every other part of the prophecy also.
Look at G.R.RM on how he writes. He goes out of his way to make the predictable tossed aside. He likes the unlikely hero, and Jorah would be unexpected and he fits all but the has the blood of the Dragon so far. He is acually a better fit than Jon Snow at the moment.