The only qualifications I would make to the advice given, is to pass on advice given to me by my past and present supervisors. If you do find you have a genuine passion or strong interest in a specific focus then this absorbing interest can often help sustain you through the difficult periods that come up throughout the PhD. And, while you may begin with the broad outlook, you will need to be thinking about the gaps in the research or an interesting problem or event you wish to explore, so starting to think (even in very sketchy sorts of ways) about this might assist you in finding a focus if you need to write a proposal or application.