That’s right, you can upgrade from the Kentico 12 MVC development model to Kentico 13 with the instructions you mentioned. Moving from .NET Framework 4.8 to .NET Core may be a separate upgrade later.
Note that the Kentico 13 CMS runs on .NET Framework 4.8 in both cases. For shared projects between the CMS and your .NET Core web project, you can target .NET Standard 2.0.