Seems like a simple bug. After researching Preservation of Knowledge as Abbassid, the cost of Dock Arrowslits is reduced from 75w/125s to 52/87s
Here’s a screenshot to show this. I reproduced this in a single player lobby.
technically it’s labeled as “naval technology” so I think it’s correct that it’s getting discounted. But I agree it makes more sense to have naval arrowslits be an emplacement.