They're just cosplay horns, it doesn't matter ultimately. At the very least, with cones, you can see the arrow shape from all angles which gets the point across nicely.
At first I was like "Wow you are being such a dick" and I still think you're KIND OF being a dick (KIND OF. not actually, but KIND OF) but all in all you were just trying to make a point but you could've worded this first comment better at the very least so it sounded more like constructive criticism and less like you picking on it. C:
Obviously someone doesn't know what a prism is. Troll horns don't have to be round at the base you know. And they could get narrower at the top to shape an arrow. Plus his horns represent a broken arrow so you can't really deny this fact :3