Recommendation engine is a generic term: https://en.wikipedia.org/wiki/Recommender_system
Dan made a post about the specific algorithm he used, but I wouldn't guarantee that the post matches the code. The code is the only real specification.
Because of the tension between the monetary incentives and the utility incentives of the recommendations for one thing. For another, the recommendation system in Steem was't addressed in my post. I think it probably didn't exist at the time, but I don't precisely remember (the post includes some comments about the concept as a potentially viable direction to explore which suggests that it didn't).