Yes but the code was clearly derived directly from a decompiled binary; not ‘clean room’ reverse engineering. Hence, illegal, regardless of whether a dmca takedown notice is filed.

Deriving something from a decompiled binary isn't illegal in itself.

That still leaves it as a derivative work, which is protected from copying and distribution by copyright.

That doesn't make it illegal, there are plenty of jurisdictions where non-clean-room reverse engineering is perfectly legal.

But distributing it without a license from the copyright holder is.

Distributing what, exactly? It required you already owned a copy of the game to install/build it.

Distributing whatever was in the repo. Requiring a copy of the game doesn't magically make it legal. A modded game is a derivative work.