Doesn't look like the author knows what he is talking about. His point about early stage startup should not use K8S is fine. But the next advice about not using a different language for frontend and backend is wrong. I think the most appropriate advice is to choose a stack which the founding team is most familiar with. If that means RoR then RoR is fine. If it means PHP then PHP is fine too. Another option is to use a technology which is best suited for the product your are trying to build. For example, if you are building a managed cloud service, then building on top of K8S, or FireCracker or Nomad can be a good choice. But then it means you need to learn the tech being used inside out.
Also he talks about all of this and then gives example of WhatsApp at the end. WhatsApp chose Erlang for the backend and their front end was written in Java and Objective-C. They could have chosen Java for backend to keep frontend language same but they didn't. They used Erlang because they based their architecture on Ejabberd which was open source and was built with Erlang. Also WhatsApp managed all their servers by themselves and didn't even move to managed cloud services when they became available. They were self hosting till FB acquired them and moved them to FB data centres later on (Source: http://highscalability.com/blog/2014/2/26/the-whatsapp-archi...).
Also he talks about all of this and then gives example of WhatsApp at the end. WhatsApp chose Erlang for the backend and their front end was written in Java and Objective-C. They could have chosen Java for backend to keep frontend language same but they didn't. They used Erlang because they based their architecture on Ejabberd which was open source and was built with Erlang. Also WhatsApp managed all their servers by themselves and didn't even move to managed cloud services when they became available. They were self hosting till FB acquired them and moved them to FB data centres later on (Source: http://highscalability.com/blog/2014/2/26/the-whatsapp-archi...).