As a C# developer (and newbie Pixinsight tinkerer), I can tell you that although .NET Core is cross-platform, it can only be used within an application targeting one of the .NET runtimes. Pixinsight is built with QT, so has no relationship with .NET and the development language is c++.
The authors of Pixinsight could move to Xamarin for cross-platform apps (in .NET - C# or C++) which would avoid the limitations of QT (especially in windows 10 high dpi systems), but it would require a complete re-write of the application.
Hope that clears it up a bit