NuGet Server for .NET Core
Yes, this is the answer:
No .NET Core whatsoever! Just .NET Framework 4.6.1. Not a big discovery but a warning to all devs who, like me, were (and, more importantly, will be!) misled by 'Core' in the package's name.
Nuget.Server.Core seems to be one of the dependencies of Nuget.Server. So it's not a .NET Core project. Guess that's one of those snags you run into when you call your framework "Core". :)