Apparently, he's criticizing the movie because it was historically accurate!

What a silly thing to say.
D.W. Griffith's Birth Of A Nation is an important film for a number of reasons,
but historical accuracy is hardly one of them.
The film portrays the KKK as kind, decent, heroic men, and negroes as idiots
REGARDLESS of slave status. It is widely criticized for this flaw, but also
widely accepted and understood to be an important film. D.W. Griffith thought
the way a lot of people still think to this day. Doesn't make his work useless,
but flawed in some very distasteful ways.
By the way, my great grandmother was actually IN Birth of a Nation, so I became
somewhat familiar with this film from an early age. Even as a kid I knew what
was inaccurate and racist, and what was fascinating and accurate, about it.
Linus