Earlier quoted context omitted.
I assume they aren't, because their Resharper codebase already had the bits needed. Still, if you start writing a C# IDE from scratch, wouldn't the obvious choice be using Roslyn for everything from autocomplete to refactorings? Easily half to 90% of the work going into an IDE has to be in the language services (if you want to do the really tricky things well, like completing broken syntax).
It's not built from scratch. It makes full use of the existing ReSharper code base.
R# is written in C# while their IDEs is in Java. Most likely they "convert" some of the C# code to Java. It'll be ugly...