If that's what he referred to himself as, and that's the only name he was known by... why not?SHatner addresses that in his book ("Collision course") and I like his solution...
Kodos took over the colony and declared himself governor... AFTER the crop-destroying plague. Nobody knew who he was, because he was a "masked terrorist" type who seized control by force.
Then it seems strange to refer to him as Governor Kodos 20 years later.