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).
"instead of reimplementing ReSharper’s features on the IntellIJ Platform, which runs on the JVM, we’re using ReSharper in a headless mode, out of process, and communicating with it via a very fast custom binary protocol. "
