How to decompile an "invalid" .net library?

Question

.NET

I bought the game Dungeon of the endless written in Unity. I bought it on Steam, so it integrates with it. This is usually done through Valve's native c++ library, but these developers seem to have decided to write their own wrappers.
When trying to decompile it, ILSpy and dotPeek unanimously declared that this is not a .net library, although ILSPY kindly reports that this library is under .net 4.5 (!)
Question: is it possible to somehow get into it, or is it such a clever obfuscation?
Actually, the library.

Answer 1

This is not a .net build.