Kind of obvious when a DOM/Web API is mentioned, isn't it? Besides, does it really matter? Same ideas you use for building a frontend router can be used to build a backend router, and vice-versa.
Maybe not so much for an Internet Protocol router?
The actual title, and the original HN story title, was “Build Your Own Router”, which, in a general tech context like HN, I would assume mean an IP router.
Sure, but once you open up the article and see browsers with green checkmarks next to them, and "this feature works across the latest devices and browser versions", then most people have to be getting what the coming body text is about, right?