I believe the usefulness of this is that now the _server_ knows the client is headless. Then blocks it.
The techniques explained in the article seem like they'd be JS running on the browser itself, so… the "browser itself knows it's headless" pretty much sums it up.
Isn't it more like, "the browser can be coerced into revealing to the server that it is headless"?
But for that you need to send content first? That doesn't work before you send the result (unless you want to redirect each request)
Utilizing javascript on browser, the server can detect it's headless.
The server can then block, redirect or feed the browser erroneous data.
Server can't do anything. It wasn't informed. And it can't be informed reliably.