I doubt very much. They're basically using Redis as a much more powerful memcached. SQL Server still is the ultimate owner of all StackOverflow data, last time I talked to the team.
I guess I just haven't seen memcached integration in .NET. All of the caching examples I've seen use the framework caching components.
I was also wondering if they'd developed any generic custom code to fetch from Redis or fetch from SQL/insert into Redis, and how it fits into their application logic. (e.g. does it happen before they make a LINQ to SQL query, or does it intercept LINQ to SQL queries and return results from Redis).
And this page has caching web services examples (at the bottom), with source code of non-non-invasively dropping in a Redis cache provider using an IOC.