C# isn’t tied to Microsoft. Unity for instance, uses Mono.
And don’t get me wrong, I got my modding start in UnrealScript as well with Unreal Tournament. However, back in those days Epic Games provided some C++ source as well. Mods could provide DLLs etc. And it wasn’t gimped like the UDK version, the native DLLs were directly tied into the engine (unless my memory is failing me). I stopped modding somewhere around UT 2004 and went into games industry where we able to use a debugger with UnrealScript. But the community didn’t get the debugger with UDK for whatever reason (only licenses).