The example LibA/LibA.csproj creates a NuGet package that incudes both LibB and PrivateLib in a single package.
Furthermore, only LibB is visible to consuming projects. This is done by explicitly copying assemlbies to the ref/ folder. If you don't want this behavior, omit all ref/ entries in the nuspec so all assemblies in lib/ will be used. In MSBuild 15.3 / Roslyn 2.3.0 the ref assmeblies could even be generated using the reference assembly generation capabilities of the compiler, resuling in smaller binaries in the ref folder.