It's an interesting question. The pertinent entry in the Random House dictionary on horror says it is ... "centered upon or depicting terrifying or macabre events:
a horror movie"
Films like
Crank,
First Blood,
Walking Tall,
The Punisher*, etc. all feature revenge motives, but are clearly not horror. They almost universally fall into IMDB's "quadfecta" of "Action, Crime, Drama, Thriller."
Yet a film like
I Spit on Your Grave certainly has a terrifying aspect to its intensity, and a macabre aspect to its story. IMDB judges accordingly ... "Drama, Horror, Thriller."
I think it's largely a personal distinction as to what would make a "revenge" film into "horror." Is it the execution? Is it the style and intensity? Is it the brutality, or the deliberate graphic nature of individual scenes? Certainly
Irreversible is intended to disturb, whether most viewers would be disturbed in a "terrifying or macabre" fashion is going to vary from person to person.
But as has also been pointed out, just because it's a Horror marathon doesn't mean that fringe films can't be included, especially if it's something seldom seen, innovative, etc.
I was happy enough to see it shown. And hey, Monica Bellucci's hot.
*NOTE: This is not meant to be an assessment of the quality of these various films, merely an examination of the commonality in their story and dramatic styling. If, indeed, they even have a story. Or a style.

_________________
David A. Zecchini; Creature of the Wheel, Lord of the Infernal Engines
"Damnati Im Ludum" (
VitruvianZeke@att.net)