Other than their front-end build process, Node is also used in their user-facing servers. Basically, when your Netflix app makes a HTTP request to fetch any data from its backend servers, the request first hits a Node JS server.
Netflix is behind and uses Falcor(1) for data fetching as an alternative to REST. The backend for this is in Nodejs(2). (There could be other backends in Java too though)
Netflix uses Node.JS in the build process for their frontend. They also use React.JS inside of one of their TV applications, using a custom renderer (that isn't backed by HTML.) They are definitely heavy users of JS.
When people say they use Node for frontend, it generally means for things like Webpack or Gulp, not something running on the end user's computer.
Can someone who knows what they're using node for go into a little more detail?