Genius designation is a little subjective. Some recipients of the MacArthur "Genius Grant" are true morons.
Regarding film I put many people in the genius category - Buster Keaton, Leni Riefenstahl, Orson Welles, Sergei Parajanov, Raoul Ruiz, Zbigniew Rybczyński, Ennio Morricone...
I'm not exactly sure of the reasoning either, but I suppose genius indicates someone who is very talented, original, and perhaps influential. I think a film genius will have exceptional and unique creative vision combined with the ability to deliver on it.