I imagine parent is being a bit tongue in cheek. A syndrome is any group of symptoms that seem to occur together. Often something is called a syndrome before its cause is determined. However, even after a cause is found or if it was known, the name often persists. E.g. AIDS was named before it was traced back to HIV. It persists as the name for the late stages of an HIV infection.
So while it is often the case that when a syndrome is initially named its cause is unknown, the two are really referring to different levels of abstraction of a disease. The syndrome is its effects, irrespective of whether we know its cause.
Edit: one usage is for a set of commonly concurrent symptoms, possibly with no known cause. Another common usage is a set of concurrent symptoms with multiple known causes (e.g. toxic shock syndrome).
So while it is often the case that when a syndrome is initially named its cause is unknown, the two are really referring to different levels of abstraction of a disease. The syndrome is its effects, irrespective of whether we know its cause.