What am I missing?

[Hai-Etlik]

Because the earth is a sphere, it actually doesn't matter where it is in the world, because it is an regional map.

Yes, it does matter. At the equator, the "box" bounded by a small equal angles of latitude and longitude is approximately a euclidean square. As you move into higher latitudes, it narrows and becomes more like a trapezoid, until you get a very narrow triangle at the poles.

A map in a projection suitable to a restricted extent is going to try to minimize distortion within that extent so those shapes will be approximately preserved: At mid latitudes, a lat-long box with equal angles would be approximately a rectangle with an aspect ratio determined by the latitude. In a normal cylindrical projection it will be projected as precisely a rectangle.

In tangent normal equidistant cylindrical, all such boxes are squares, but this means that they, and all other shapes, are being stretched out east-west. Which is a rather noticeable form of distortion. So my point stands, to get a square lat-long graticule, you can either be covering a small area near the equator in a projection that minimizes distortion within your extent. Or you can be in Plate Carree, in which case you can be anywhere, but would have significant and ugly distortion unless you are covering a small area near the equator.

Plate Carreé projections (also called "equirectangular") were also invented around 100 BC, so having one exist in the middle ages actually is completely valid. In fact, the Mercator map, one which is usually imagined when you think about old maps, was invented in the mid 16th century. Latitude and Longitude were also invented by the ancient greeks, as evidence by this map made in the second century.

Although this map is inaccurate, Latitude and Longitude were, in fact, used.

Yes, I know the ancient greeks had an idea of latitude and longitude and tried to use it in map making. My point was that maps from the Medieval period, which high fantasy is generally based on, didn't. In fact they rarely used maps at all let alone ones with graticules. When they did draw maps, they were at best about giving a crude idea of topology: what was connected to what. That's why I specifically said "a bit anachronistic" I admit that wasn't exactly the clearest way to express my point. Dropping the graticule (admittedly somewhat harder given it's hand drawn) would sidestep the whole projection issue and better give a pseudo-medieval, high fantasy feel.