Don't take my word seriously; I just watched things as they happened.
Back when Microsoft extinguished XNA (circa 2012), people were already using Mono to write C# applications for non-Windows platforms. (Don't confuse MonoGame and Mono!). Unity used a version of Mono to allow developers to write C# code for their games. You can read up on it more but Miguel de Icaza was the primary developer for Mono. He is also the founder of Xamarin. There is a lot of experience with developing for multiple platforms right there. Over the years, this knowledge and technology have made it back to MonoGame one way or another. Things changed. SuperGiant Games forked MonoGame for development of Bastion. Microsoft acquired Xamarin. Miguel now works for Microsoft. Microsoft acquired GitHub... The story continues, but that is just one side.
Another side of the story is that Tom Spilman (lead of the MonoGame project) and the rest of Sickead Games has a lot of experience helping developers who developed their game on Windows port the game to consoles. In fact, if you check their website it lists that one of their services is to port games.
Together, the story answers your question. Over the years, from rising to meet the need of a disbanded API to being open-source, MonoGame has acquired a lot of knowledge about making games for multiple platforms and continues to exercise that knowledge.