You are right, its a tough one, many large websites do deploy this type of stratergy as it provides the right files for the right device, however, for those just developing a simple website, its an easier and more cost effective route to just go responsive, you then know that your website will look good on all current devices as well as future ones.
The thing is, RWD does not need to pick up mobile user agents because it works as is. We live in an age where opera is the norm. And as stated in the article, thats just fine, we can work with that. RWD can work with that.