.NET case was mostly a rewrite, many components were rewritten from scratch (sooner or later). Roslyn was a compiler-rewrite to make C# self-hosted with, what I understand, intention to make it OSS from the beginning. .NET (then Core) started with RyuJIT. AFAIK RyuJIT also mainly exists for .NET only, where-as .NET Framework uses older, different compiler codebase.

But yes, there were additional licensing assurances provided so as to not create problems for Novell/Mono - it was always treated positively.