There's different kinds of ahistoricity that you can complain about. I'm far less bothered than the author about the way AC works (although the end result that everyone ends up around the same AC does raise an eyebrow to me), for example. But the opposite spectrum is introducing something like studded leather armor which is... why? It's an armor type that doesn't exist and that makes no sense [1], so it's not even a "well, in a fantastical setting, it might make sense" sort of scenario.
The other issue is that this kind of fantasy is often the way by which people learn about history. This means that when fantasy engages in very ahistorical treatments of things with potential historical analogues, people will assume that the fantasy treatment is realistic. This definitely applies to armor--people will look at the armor types in a fantasy game and assume they're at least broadly reflective of how armor in Medieval Europe worked, since it's not overtly coded in fantasy unless it's explicitly dragonskin or something like that. So when DnD is treating leather in a completely different way from how it normally works... that's a constant canard that historians are continuously having to try to rebut (much like myths about full plate meaningfully restricting movement).
[1] Just to remind you, studded leather is a misinterpretation of images depicting something like brigandine armor. In brigandine, the apparent studs have value--they're the attachment points for metal plates. But the entire point of studded leather as an in-universe armor type is that it doesn't contain these metal plates, so the studs serve no purpose.