I did a little digging and though the article does not name the other providers, based on pricing, I believe Hosted AWS Provider #1 is MongoHQ [1] and Hosted AWS Provider #2 is MongoLab [2]. (MongoHQ no longer has Replica Set: Small on their Pricing page but I found it in the Google cache. [3]).

[1] https://www.mongohq.com/pricing

[2] https://mongolab.com/products/pricing/

[3] http://webcache.googleusercontent.com/search?q=cache:cj8DBbH...