I would read this as meaning that if you id an aircraft these are the methods that can be used. Not that you have to id the aircraft, or to do so using SSR.
You could, for example, id the primary target (if you have one).
But as to your original question, unless you have rules to the contrary where you work, the controllers job is to separate the aerpoplanes using any suitable and valid separation standard. Imagine if you only have one aircraft, why do you need to use radar to provide separation? Or two aircraft that are vertically separated?
However, separation is only part of the controller's job and having surveillance and accurate positional information is always better than nothing more than an estimate.
