Yeah, imo the term "game engine" itself has kinda evolved over the years from "framework that contains common game/renderer code" to "platform to handle the entire process of implementing a game".

It's just code in the end, so even the "full platform" option can be used as if it's just a framework, which it sounds like RimWorld was doing if it could switch engines without breaking mod support (another comment guessed LLMs were involved but IMO this goes way beyond what an LLM would handle well, as it would involve pretty much reimplementing unity's framework). Though I don't have any direct experience with Rimworld mods and how exactly they interact with the game; if they used a data-driven approach, then it could be possible to redo just that interface and keep mods working (though it would also depend on what exactly was running the mod code itself, as that part would also need to remain functional).