In a paper that I read recently, the authors promote their software as open source because it was released on GitHub under the Apache License 2.0. However, the software is an add-in to a proprietary (closed source) and non-free software. Furthermore, it was developed using C# and the .NET Framework, which I think should, historically at least, has not bebeen considered open source or free software. Based on this, is it really correct to label this add-in open source software?