The article compares Redis to Cassandra which means comparing apples to oranges for their use case. It would have been meaningful if they explained why they chose Casssandra, instead of for example Hbase.
According to the article, the interviewee used to work at DataStax (leader in Cassandra) before joining Instagram. I guess this is one of the reason why they first considered Cassandra and it turned out to be very successful. Biased or not, it is always good to have an engineer with deep understanding on the database, rather than just being an ordinary user.
