I think "skribisto" would refer to someone whose job is the action of actually writing - putting words onto paper, in the same way that "tajpisto" is someone who types. Neither means that the person concerned actually decides what to write or type, as an author (verkisto) does.
To create or make work, in English, means to do something which means there is work to be done, either by oneself or by someone else. For instance, if a child eats a meal carelessly, and spills food down his clean clothes, someone has to wash those clothes. So "work" there doesn't imply plural. It's a mass noun.