I feel like you misunderstood the OP, they are claiming that Node allows you to reuse the same code to do validation on both the client and the server. By definition that means they are also doing server-side validation, and they are not relying on it being checked on the frontend.
As I see it though, node.js on the backend is not mainstream, most sites are still using JVM or other back ends. Using the same code for the front end and the back end is a dream that has been pursued in various forms but it isn’t mainstream.
Man... if you don't think node.js on the backend is mainstream at this point I don't know what to tell you. It's not even the hyped-up new thing anymore.
Being not hyped up doesn’t mean it’s mainstream. Most backends are in Java, Go, or PHP. Python and Ruby take up most of those that aren’t. It’s rare to find node on the backend in comparison.
