I think you have to read to the part where he talks about the deployment impacts. A key part of what happened was they needed deployment flexibility as well. From their perspective, they would need to implement the same basic infrastructure to achieve that flexibility:

But even if everything went smoothly, we knew that the current code for the monolith had to be refactored anyway. The code had suffered a lot during the past few years, tech debt everywhere. Besides the mess we made ourselves, we still had to update it from Rails 2.x to 3, and this is a big migration effort in itself.

This was probably critical their ability to adopt new technologies, like Clojure and later Scala.