I didn’t state that BIrth of a Nation was the first long film. I said it established the long form drama. The running time was nearly twice as long and ticket sales ten times more than those earlier films.
The reason that BOAN is still remembered, even by people who know very little to nothing about early film history, is because it was/is filled with lies, is blatantly racist, anti-North, and was shown in the White House by then president Wilson and highly praised by him.
There were quite a few SIX reel movies, made by others, a year or more than Griffth's racist, controversial, alternate history film. And it was too long!
OTOH...his INTOLERANCE would have sold your case. ;^)