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Not a lawyer, but I'd expect that decompilation creates a derived work which is still subject to the copyright of the original.

If you engineer a clean room process, it does not. Already proven with an open source project that went from GPL to MIT via clean room (chardet).

Exactly this. Using an LLM to study the decompiled output then another LLM to implement an emulator/engine/etc

That sounds similar to how LLMs have been trained on GPL code and yet output is not (yet) considered derivative work