I don't think Xamarin has open sourced anything that they didn't have to. All tools developed exclusively by Xamarin are closed source (e.g. Xamarin build host, the Mono packager, Xamarin.Insights, Objective Sharpie, etc...).
Xamarin contributed several projects to the .NET Foundation. Mono is also permissively licensed, so every contribution Xamarin makes that does up in the open (instead of locked up, which they could do by actually taking advantage of the terms in those permissive licenses) is something "that they didn't have to" do.