It doesn't sound that Stevens was denying that that stuff actually happened, just questioning whether emphasizing the sheer gruesomeness/violence/sexual horror of it is the best way to communicate the intended message. I haven't seen the movie, so I don't now if that is at all true.
The point of the gruesomeness is to emphasize the horrors of slavery.
I realize that. I think that the point that Stevenson is trying to make is that those tactics (that is, emphasis on physical violence) might not necessarily be the best way to get the viewer to consider the theme(s) of the film.
Why not, exactly? To emphasize the horrors of slavery, the movie emphasizes the gruesomeness of it, which is actually realistic in this case. It's not necessarily exaggeration; the goal is to show how awful it really was, no holes barred.
As I said, I haven't seen the movie, so I can't affirm or deny Stevens's claim. However, I do think it is possible for a film to focus too much on "guts and gore" at the expense of the deeper meaning, even if said gore is realistic.
If the film was intended to be a documentary, then making it as realistic as possible would be the only concern. However, if it was intended to be a deeply meaningful film (as I'm sure it was), then potentially distracting elements (such as an excessive focus on physical violence or quasi-pornographic scenes) might be worth considering removing.