You’re asking for an argument with a question like, “What was the first horror film?” Other questions automatically follow. What is a horror film? What is a film? Who are you?
Incubus (1966) was a commercial failure on a level that sounds made up. An independent movie that doesn’t get any distribution at all isn’t rare or strange, but an American independent movie that got distribution in one single market, France, is pretty special. It can seem a bit strange that a legendarily cursed horror film couldn’t catch anyone’s attention.