You need to use the source code in
vs2010-net-4.0 folder. It targets .NET 4.0, .NET 4.5.x, .NET 4.6.x, .NET 4.7 and Mono (3.0 or higher).
vs2008-net-2.0 folder contains source code targeting .NET 2.0, .NET 3.0, .NET 3.5 and Mono 3.x (in .NET 2.0 mode).
The P/Invokes to Windows native libraries are only used when running on Windows.
As a side note, we are currently working on adding Mono support to .NET Standard 1.5 version of Rebex DLLs. This might be the preferred option because those DLLs are also going to support .NET Core (on Linux, Windows and macOS). If you would like rather use this, please contact us at firstname.lastname@example.org.