They used the version of OpsManager that doesn't manage the deployment - is specifically not a deployment manager. Mongo does offer a managed version of this software, which the author mentions - with a justification for why they couldn't use that offering. However, I think this was the main mistake that The Guardian made. As the author notes: "Database management is important and hard – and we’d rather not be doing it ourselves." They underestimated the complexity of managing database infrastructure. If they had been attempting to set up and manage a large scale redundant PostgreSQL system, they would have spent an enormous engineering effort to do so as well. Using a fully managed solution - like PostgreSQL on RDS from the beginning would have saved them time. Comparing such a fully managed solution to an unmanaged one is an inappropriate comparison.
Full disclosure - I used to work at MongoDB. I have my biases and feelings w.r.t the product & company. In this case I felt that this article didn't actually represent the problem or it's source very accurately.