You've misread it. Here's those lines with a bit more context:
> But if, instead, they followed the peak of the energy efficiency graph, the entire chip would consume just 8.5 watts [...] and operating at about 0.4 volts, Ditzel said, one chip would take about 20 watts.
"the peak of the energy efficiency graph" isn't stated explicitly, but is at around 0.32 volts.
Basically they want to use up all 120W of a PCIe slot, at the highest efficiency possible. They could have gotten higher efficiency (at 8.5W per chip), but that would have resulted in not being able to use the full 120W and thus actually having worse performance overall, even though it's more efficient.
> Basically they want to use up all 120W of a PCIe slot, at the highest efficiency possible. They could have gotten higher efficiency (at 8.5W per chip), but that would have resulted in not being able to use the full 120W
Right, at the end of the day they were constrained due to not being able to fit more than 6 chips on a single PCIe card. If they could fit 14, they would've been able to make each chip use as little as ~8.57W while still using up the 120W available to the card.
> But if, instead, they followed the peak of the energy efficiency graph, the entire chip would consume just 8.5 watts [...] and operating at about 0.4 volts, Ditzel said, one chip would take about 20 watts.
"the peak of the energy efficiency graph" isn't stated explicitly, but is at around 0.32 volts.
Basically they want to use up all 120W of a PCIe slot, at the highest efficiency possible. They could have gotten higher efficiency (at 8.5W per chip), but that would have resulted in not being able to use the full 120W and thus actually having worse performance overall, even though it's more efficient.