Couldn't be more wrong. Mono is the ancient obsolete tech (still used in Unity but not really since IL2CPP exists). .NET Core was absorbed into .NET 5, which is the current universal framework (it's also heavily optimized and cross-platform, hence Mono obsoletion).

To OP. Man, who cares if some tiny piece of this game uses .NET Framework? It's probably used by some auxiliary stuff like launchers etc. The game itself is most likely written in C++ with some scripting solution. Which may as well be a visual scripting. So I highly doubt C# is used for the game itself.