I believe Lady Rae was responding to your original point.Well, if theoretically a high carb, low protein, zero fat diet correlates with the fastest metabolism, then I'm not sure if that is desirable nor the ideal state. It can't be, we're not meant to eat zero fat.
You mentioned that low protein >> lower thyroid function (which I hadn't heard before).
But then contrarily, Lady Rae mentions that reducing methionine (and by extension, most likely reducing protein, unless all of your protein is from very low methionine sources) actually raises the metabolic rate.