There is no statement in our grammar that was ever meant to imply that "a god" was a permissible translation in John 1:1.
The above statement is plenty dishonest for a scholar. I did my own research in finding the same greek structures and it is clear that, depending on the context, you can interpret the anarthous as indefinite. Of course, I am not saying as the Watchtower that we must translate it as "a god", but it is not grammatically incorrect.