Well, you're blaming the French for a decision made by a Kiwi
Unless you're a specialist neurologist, it doesn't really matter what your opinion was, even less so Jonny or Nige. The TJ saw something and called for an HIA. At that point only Nige is able to countermand it, and there is absolutely no way that he would do so.
Nige never spoke to the French Doc.
Nige was also being wrong in this instance. The HIA was called by the TJ, and relayed onto the field by Nige, who knew full well who made the call. It's out of the doctor's hands at this stage, and rests entirely with Nige.
So, how come it's impossible for someone to injure both their head and their knee? The entirety of your argument lies on this impossibility. If it is possible or, the the damage to his knee is utterly irrelevant to any potential head damage.