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As the OP notes, MS use their own fork of Mono for the .NET Core runtime these days. So you're not wrong but they are different codebases these days.

The .NET Core runtime was never a fork of Mono, it’s always been a separate code base.

I never claimed that the .NET Core runtime is a fork of Mono but it certainly contains one: https://github.com/dotnet/runtime/blob/main/src/mono/mono/mi...