If you look at someone from eye level it is rather hard to mistake their head for a football. You need a certain amount of elevation before it actually starts to look like a sphere.

Yes, this.

Even looking like half a sphere is enough, as the system attempts to detect partially occluded balls too.

To be fair, he did wrote "ball" not "sphere". I'm pretty sure my head and a ball share a lot of roundness but my head has way more parts that are sticking out. Also has way less of pentagon patches on it.