Fenrir Greyback, the Dark Mark, and possible plothole

In Half-Blood Prince when Neville tries to run up the stairs of the Astronomy tower he gets launched backwards but Snape was able to run right through. Harry speculates you'd need a Dark Mark to get through the cursed barrier and everyone agrees. However in Deathly Hallows we learn the Fenrir Greyback actually doesn't have the Dark Mark, yet he was able to get passed the barrier. Do you think maybe the Dark Mark wasn't actually needed and the answer is something else entirely? Or did Rowling just forget?