I read this line as "There will be backward compatibility between DotNetCoreAppVersion.V5_0 and FrameworkVersion.V4_8" and that is probably not true

From the design document:

net5.0. This TFM is for code that runs everywhere. It combines and replaces the netcoreapp and netstandard names.

Also, under design, it mentions that net5.0 will be compatible with net1..4 with a NU1701 warning.

net4x and earlier will continue to use .NETFramework as the TFI. This means that net4x and net5.0 aren't considered compatible by default, but the compatibility will continue to be provided by .NET Framework compatibility mode we introduced in .NET Standard 2.0. It's handled via AssetTargetFallback in NuGet restore which also means consumers from net5.0 will continue to get a proper warning.

Please note that net5.0+ will map the TFI to .NETCoreApp. We need to announce this change so that package authors with custom .props and .targets are prepared.

Is .NET 5 a superset of .NET Framework 4.x?
No, .NET 5 isn't a superset of .NET Framework. However, .NET 5 is the successor of both .NET Core 3.x as well as .NET Framework 4.x.

Basically, netcoreapp5.0 is a temporary TFM that will go away (the code to change it to net5.0 has already been merged and it's being discussed whether it should be backported to preview2 (from preview3)).

thx.