I was speaking in the sense that the base game is free and you can buy it with non-ascii graphics for a price.

However,

The "raws" that drive the game are completely configurable and accessible by a user.

It's more like the engine being closed source and the gamedata being source-available. Modding isn't quite the right word - that implies it being less open.

You can delete stuff from being present in your game, add new plants or objects, new diseases, etc.

Also related, the game has been opened up with Lua scripting thanks to Putnam's efforts, for even more powerful procedural addons.

https://dwarffortresswiki.org/index.php/Raw_file