IIRC I believe Clarke is quite clear that the novels (including the novelisation) are not intended to be the definitive and only explanation of the events of the film; I presume the sequel film - based on one of the novels - is included in this assumption. 2001 is an intentionally open ended film.But the sequels and the novel are pretty clear that the monolith was created by an alien race, so many people tend to interpret the first movie with that in mind.