It is the runtime from the Silverlight Project, which evolved over the years for their Windows Phone and later UWP platforms. .NET Core is a very old platform already. It shares the jitter and the garbage collector with the full framework.
It is actually the opposite of your statement. Mono is massively adopting .NET Core source code but without loosing their own benefits (portability and factoring).
Microsoft open sourced Dotnet to have others port it to different platforms and fork the code not rename it. Big difference between just renaming something and forking it under a different name.
It is actually the opposite of your statement. Mono is massively adopting .NET Core source code but without loosing their own benefits (portability and factoring).