Patina is not very easy to define.
In some circles patina is seen as a (green) discoloration of metal, either caused by time, or by chemicals.
In ethnographic collecting patina is seen as all changes that come to an object during its life time. This also includes dirt.
For example, I can not imagine that you would take an old fetish statue from Congo and decide to gently remove the dirt in order to reveal its true patina
In this case my choice would have been to leave the dirt or at least some of it.