Right, but the problem with having the narrow part on the inside is that any arrow that hits within the "cone" will be "funneled" right into the defender; whereas what you want is for attackers' arrows to hit only if they manage to get just the right place.

The wording is slightly unclear, but I can't imagine anyone this well-versed in medieval warfare wouldn't know that arrow slits would be wider inside than outside.

I do think the person above you is correct in that the author of the blog made a mistake. I've never seen arrow slits that are narrow inside the wall.

If I look at the old wall around my city's medieval centre we can clearly see they this:

From the outside: https://www.google.com/maps/@50.845463,5.6946097,3a,75y,330....

Inside: https://www.google.nl/maps/@50.8456076,5.6945364,3a,75y,165....

Huh. It looks like you are right. The narrow inside don't actually impede the view of the defender that much. But keeping the narrow part on the outside does provide some extra defense.