I think you're misunderstanding what they mean by "static." The served pages are static html and javascript, and the javascript loads other resources from the server. The "static" part just means there is no page rendering occurring on the server.
Out of all the processes that must occur to render a dynamic server-side response (database queries, computations, etc.), assembling HTML ("page rendering") is probably one of the least expensive. This seems like a premature optimization to me.