That's the official line but I honestly don't know anymore. We were trying to write a few cross platform libraries that we could also consume in Framework. We could not get .NET Standard libraries to import into Framework Apps without entering a dependency hell that made me long for the DLL Hell of the 90s. We discovered that re-targeting .NET Standard projects to .NET Core would allow them to be imported into .NET Framework projects without issue.

Of course you can't import .NET Core libraries into .NET Standard so in the end we actually made projects for each of them with links back to the .NET Standard libraries' files.