The second link in the post you replied to explains how to use NuGet packages without using a .csproj file. You are not limited to in-the-box libraries. Not having a .csproj is the whole point of this feature; everything in this blog post is talking about things you can now do without a .csproj file.

Repeating the link again here: https://devblogs.microsoft.com/dotnet/announcing-dotnet-run-...