I see that statement surprisingly often enough. Whenever I talk to high school students who are interested in doing physics, or even new undergraduate students thinking of majoring in physics, I often ask what they would like to eventually go into. The response I get is of the type "I want to do theoretical physics". When I ask them what they mean by "theoretical physics", I often get a reply that to the effect that they want to study string theory, elementary particles, etc.. etc. In other words, to many of these people
theoretical physics = string theory, elementary particles, and that type.
This, of course, is a highly faulty understanding of what "theoretical physics" is. It is no different than this very poorly written "guide" on becoming a physicist.
For better or for worse, physics has many different fields of study. If you look at the various division of the APS, you will get a good overview of all the different areas of physics that currently covers most, if not all, of the professional physicists in the US. So these are the different types of physics that people are working on. But also note that, in many cases, a person could be working in more than one field of study, i.e. the work involves more than just one field.
Now, within each field, we have both experimental and theoretical areas, well, all except string, which has no experimentalists! :) So if you are working in, say, nuclear physics, you can be either an experimentalist, or a theorist. Even so-called "applied" field, such as condensed matter physics, accelerator physics, etc., you can have both theoretical and experimental work.
So what this means is that, if you say you want to do theoretical work, that's rather vague and puzzling, because, it means that you haven't made up you mind what area of physics you want to work in. That's similar to someone saying "oh, I want to do experimental work", and someone would then reply "yeah, but doing WHAT?" Now, it's OK if what you mean by saying such a thing is that you don't quite know what field you want to work in, just as long as you are doing theoretical work. If this is really what you intended, that's fine. But most of the people who claim that they want to do "theoretical physics" don't mean that. They have a very narrow view of what physics is, and more importantly, what "theoretical physics" is. I've seen a look of surprise when I told them that Phil Anderson, Bob Laughlin, John Bardeen, are all theorists in condensed matter physics (which is often thought to be an "applied" physics), and they all have won Nobel Prizes in physics!
I think this is one of the "myth" about physics (and about physicists) that I try to constantly smash to pieces. Physics isn't just the LHC, and physicists aren't just the Brian Greene's. It is also the iPods, the MRIs, etc.. etc. And for someone who still don't know that "theoretical physics" does not automatically mean what they think it means, it is highly advisable that they hold off on focusing on what they want to do before they have done sufficient "window shopping" to see what physics really is and what it has to offer. At some point, there needs to be a dose of reality injected into a decision on what one wants to do.