The whole point is that it’s impossible to create such a map in a regular editor. What mod or script was used to make it possible to create a map of this format?

You need a modified map editor.

In general, the essence of the map editor modification is simple: a part of the executable file is adjusted, in which the map sizes are specified: the standard map sizes are replaced with larger sizes. The editor itself works normally with these large sizes.

In theory, you can create a map of simply enormous size, but as practice shows, maps larger than 252x252 (XXL) have glitches either in the editor and/or in the game itself.

P.S. In the Horn of the Abyss add-on, the standard map editor allows you to create XXL-sized maps, but this size is called G there, and you can only play the maps created in the Horn of the Abyss editor in Horn of the Abyss.