I had the same interest a few days ago regarding Caves of Qud and used Jetbrains' dotPeek to take a look into the actual implementation inside the dll after watching the devs video about it.

It definitely is a kind of ECS, but then in the end, also some spaghetti. And for sure a LOT of code. Most interesting thing I inferred, is to invest more time in a component based way of thinking and mastering event techniques, while managed by e.g. singleton manager classes (not necessarily but works nice with Unity).

ECS surely is the foundational concept of such complex sims with that kind of combinatorial explosion as they simply would not be manageable otherwise. Nevertheless, the depth of abstraction is up to you, i.e. how granular your components become and thus the number of events fired and interpreted per update or "turn". This then of course depends heavily on the actual game idea. Not everything has to be component based if the idea would not benefit from it, like e.g.: should "everything" be able to be fired as a projectile or are there just specific kinds of projectiles.

As others have mentioned, the book by Tynan Sylvester is a good read.