Here is my take: A fictional, non-Earth world cannot have "anachronisms," or be "historically inaccurate," since it has no real history or chronology to adhere to.
Just because the zipper was invented relatively recently in the real world doesn't mean that a fictional civilization couldn't have figured out how to fasten cloth with interlocking projections of metal at the same time they invented catapults, or what have you. Especially one with magic.
It's useful to take the real world as a loose guide, but even then some people confuse the middle ages with existing fantasy literature, or insist on exclusively European discoveries and inventions.
Another example: some people chafe at clockwork devices in D&D, saying that clockwork did not exist until the rennaissance, or whatever. Well, let me take you to Baghdad in the 900s where the palace of the Caliph featured an entire clockwork tree made of silver and gold, complete with little clockwork birds that sang, and leaves that twirled as if in the wind.
Sounds like Eberron, almost. But this was hundreds of years before Da Vinci.