To be clear, it is really a thing. The blood in the brain and behind the eyes can be explained with violent shaking. It's just not the only explanation for those symptoms, and some innocent parents/caregivers have had to suffer due to the system regarding violent shaking as the only cause.
Actually, even the question to know whether the blood in the brain and behind the eyes can be explained with violent shaking (without impact) is debated as the scientific evidence is still lacking. I said "without impact" because the answer is clearly yes when there is an impact -- but there will also be additional markers of trauma in those cases.
So, it's not only to know whether it's the only cause, but even more fundamentally, whether it is a cause.
In any case, one can't really say that SBS is "not a thing", because babies are definitely shaken, and some are injured or die -- there's no debate about that, unfortunately.
