Arithmetic includes more than just addition.
The only operations that are valid for AC are addition and its inverse, subtraction. You don't multiply ACs by anything, nor divide them by anything. Such concepts have no meaning. Hence, they are interval data. Interval data counts upward. That's what it's for.
The one--and only--context where "count up" is bad is stuff where having more of something is worse than having less of it, not where a small value is somehow greater than having a big value. Stuff like golf, where taking a single stroke to do something is clearly a display of better skill.
AC is not such a thing. The very fact that an "AC0" can exist, to say nothing of
negative AC, is proof that it should not be structured the way golf scores are. With golf, there is a clear minimum stroke count: one per hole. AC doesn't work that way--and, as far as I can tell, essentially never did.
It is both more natural and more logical to use a data type which reflects the valid (and invalid) operations you can perform, and which marries the nature of the numbers in question to the nature of the property in question. Ordinal data does not do this, so that's out, and there is no sense in which "golf scoring" type interval data could be applied here. It is just regular interval data--which means the most natural and logical expression is to have greater values mean...well, greater defense. There is no need to make this
ratio data, because you simply don't
do division or multiplication of AC. There just isn't any need to do so, nor is there any defined sense for such a thing.
Interval data counts things of a common, discrete size. That's how AC works. +1 AC is +1 AC, whether it comes from a +1 Ring of Protection or a +1 AC buckler or whatever else. And if we're going to say that a Ring of Protection has a +N enchantment, you can bet your britches it should be the case that you ADD that amount to the AC in order to make it
better, not worse.