The code is not the problem, nobody says that. But everything which was created as part of the Diablo universe can potentially be subject to copyright law.

For example here they take the original text from the game for the labels: https://github.com/OpenDiablo2/OpenDiablo2/blob/37ae98d81b79...

For sure most of the game specific data can be extracted from the game files but it's unlikely that there are no artifacts in the game code itself.