It wasn’t a fork. It was one and the same. Xamarin was started by Miguel De Icaza [1], who also created Midnight Commander, GNOME, and Mono, the .Net runtime for Non-MS platforms. Xamarin was bought by Microsoft. Mono Develop became Xamarin IDE became Visual Studio for Mac.

I know this because I contributed. Along with working alongside the grandfather of MonoGame. In 2010, .Net was in a weird place.

[1] https://en.wikipedia.org/wiki/Miguel_de_Icaza