As someone else has mentioned, I believe it's a metaphor for Meta as a company. It's probably a little extreme at 5% but he's suggesting they're only working at that efficiency level.
For the monster hardware it has, Oculus (at least in games) can only rival a PS2 in terms of polygon count. Pair that with the 5% GPU utilization metric and you get that this device can handle a whole lot more load and it's just being used inefficiently. I'd even use the word 'wrong', people within Meta made 'good enough' libraries and called it a day, and he's mad about that.
This thing has billions of dollars poured into it, and all you get is PS2/PSP level graphics? No way, man.
Carmack of all people knows the kind of feats this machine is capable of, having worked complex optimization problems in the RAGE Engine and the old Quake engine too.
It sounded more like he meant that he is seeing their organization be inefficient at working on the right things and making progress towards VR.
And as a person who cares deeply about efficiency of resources, that pains him to see so much human resources (in this case) being wasted when they could accomplish so much more so much quicker (in his opinion) with better focus and direction.
And he likened it to the same feeling he'd have if he saw a game making use of only 5% of the available GPU, which would be that feeling of all that wasted potential.
