Yeah, itisnotlogical has the right idea. Poke around in the files if you can, you should get some idea what's up. But it's probably based on a custom engine.
For those sorts of custom engines which are in-house the way it works is sort of like DF modders actually. The "game" team works with the existing engine, and does whatever improvements they can leverage using the existing code, e.g. similar to RAW modders working around the DF system. You cleverly leverage an existing scripting engine to do things it was never designed for.
Meanwhile, the "engine" team constantly upgrades the engine, taking care not to break the scripting side of things, but when the "game" team comes across something that can't be easily "modded" into the engine, they might request a new feature to the "engine" team, and then there's a negotiation while they work out the best way to integrate the change into the system, which might change some details about how the "game" team does things. It's equivalent to Toady creating a new DF RAW token, and now some of the ways modders are doing things are obsolete, but new things can be created as well.
This separation of engine code & game scripts also vastly reduces the cost, complexity and risk of porting your game to a new platform. Because e.g. if you want to support ARM processors on mobile, you only have to recreate the "engine" component then port in the "game" logic scripts on top of that, maybe with some UI adjustments, and updates to the "game logic" can then be easily rolled out on all the platforms even though the codebase for the engine might be completely separate.